Persistent `No Fill` Error After Unity Ads SDK Integration with Custom Mediation Adapter

Dealing with persistent "No Fill" errors is one of those classic ad monetization optimization headaches that can truly test your patience, especially after meticulous integration work. Let's break down some potential culprits beyond the obvious setup checks you've already performed, focusing on the interplay between Unity Ads SDK 4.x and custom mobile ad mediation.

First, the fact that test mode works perfectly is a strong indicator that your basic SDK integration and ad unit IDs are correct. This immediately shifts focus to live inventory dynamics, server-side decisioning, and the specific handshake between your custom mediation and Unity's ad server in a production environment.

Unity Ads SDK 4.x, Custom Mediation, and Fill Rate Nuances

1. Mediation Layer's Role & Server-Side Bidding: With custom mediation, your adapter is essentially making a request to Unity's SDK, which then communicates with Unity's ad server. Unity Ads SDK 4.x heavily relies on server-side bidding and optimization. A "No Fill" often means Unity's internal ad server couldn't find an ad that met its criteria (geo, device, eCPM, frequency capping, campaign targeting, ad quality scores) *before* it even considers your custom mediation's waterfall or bidding logic (if Unity is one of the networks in your stack). Your mediation might be asking, but Unity's server is saying "no viable ad here" for the current context.
2. Geo-targeting & Device Segmentation Discrepancies: You mentioned certain geographic regions and device types. Even with a seemingly healthy eCPM floor, the actual demand and inventory depth can vary dramatically. Tier-1 regions generally have more advertiser demand and higher bids. For non-tier-1 regions, a "generous" eCPM floor from your perspective might still be too high for the available advertiser bids in Unity's network for that specific geo/device combo. Unity's ad server has sophisticated internal filtering, and an advertiser's campaign targeting might not align with your users' context, leading to a legitimate lack of available ads.
3. Ad Quality and Frequency Capping: While less common for a full "No Fill," if your app's ad quality score (as perceived by Unity's system) is low, or if users are hitting frequency caps too often (especially if your mediation is requesting ads aggressively), it can contribute to a perceived lack of inventory.

Obscure Unity Dashboard Configurations

It's easy to overlook settings, especially when dealing with multiple placements and a complex ad stack. Here are a few to re-verify:

1. Placement-Specific eCPM Floors: Beyond global settings, double-check if specific eCPM floors are set at the placement level within the Unity Dashboard, particularly for the 'Interstitial_Android' unit. Sometimes, a regional override or a specific floor for a certain ad unit can inadvertently be too high for the available demand.
2. Geo-Targeting Exclusions/Inclusions: While you've checked, sometimes there are subtle geo-blocking rules or whitelist/blacklist configurations that might have been applied at the app or placement level, potentially impacting specific regions.
3. Category Blocks: If you've implemented any category blocks (e.g., blocking certain ad content categories), these can reduce available inventory, especially in regions with less diverse ad demand.
4. Bid Price vs. Floor Price Understanding: Your eCPM floor is what you're asking for. Unity's ad server, however, is matching advertiser bids. If the highest bid available for a given impression (considering geo, device, user context) consistently falls below your floor, you will receive a "No Fill." This dynamic is particularly pronounced in regions where advertiser budgets are lower.

Advanced Debugging Steps for Live Environments

Since test mode works, the issue is likely server-side or in the precise interaction of your custom adapter with Unity's live ad server. Standard SDK initialization logs only tell part of the story.

1. Network Request Interception (Proxy Tools): This is arguably the most powerful debugging step. Use a network proxy tool like Charles Proxy or Fiddler (or alternatives like Proxyman) on your test device. Intercept the HTTP/HTTPS traffic from your app. Look for the actual ad request and response payloads exchanged with Unity's ad servers. These responses often contain more detailed error codes or reasons from the server that are not fully exposed in the client-side SDK `NO_FILL` message. You might see specific codes indicating why the ad server couldn't find an ad (e.g., "no campaigns matching criteria," "floor not met," "inventory exhausted for segment").
2. Unity Dashboard Analytics & Reporting: Go beyond just the top-level fill rate. Dive into Unity's granular analytics:
   • Request Rate vs. Fill Rate: See if the request rate is high but the fill rate is low for specific geos/devices.
   • eCPM by Geo/Device: Analyze the actual eCPM achieved by region and device type. This can confirm if your floor is simply too high for specific segments.
   • Match Rate: Some dashboards show a "match rate" which is how often Unity's ad server *found* an ad that *could* be served, even if it didn't ultimately serve due to a floor or other issue. This can differentiate between "no ad available at all" and "ad available but didn't meet criteria."
3. Increase SDK Log Verbosity: Check if Unity Ads SDK 4.x offers a method to increase the internal logging level to `Debug` or `Verbose`. This might output more internal decision-making logs that could hint at why a fill isn't happening. For example, in Android, you might look for `UnityAds.setLogLevel(UnityAds.LogLevel.Debug)`.
4. A/B Test with Direct Integration: If feasible, for a small segment of users in the problematic regions/device types, bypass your custom mediation and integrate Unity Ads directly for that specific ad unit. If the "No Fill" issue resolves, it strongly points to an interaction problem with your custom mediation adapter. If it persists, the issue is more likely with Unity's inventory/demand for that segment at your current settings.
5. Server-Side Callbacks (Postbacks): Ensure your custom mediation is correctly sending all necessary server-side callbacks/postbacks to Unity, especially if Unity is providing the ad. Inaccurate reporting (e.g., impressions, clicks, installs) can negatively impact future fill rates as ad networks optimize their campaigns based on performance data.

The key here is to get more granular data. The client-side "No Fill" is a symptom; the network traffic and detailed server-side analytics from Unity are where you'll find the diagnosis. Good luck tracking down this elusive bug!

Hope this helps your conversions!

Yeah, this is exactly what I needed. Seriously, Sophia Williams, your breakdown is incredibly clear and that's so hard to find in these kinds of tech discussions. Really appreciate you taking the time...