Is our XML sitemap generation tool just having a bad day?

hey everyone, we've got this 'Free XML Sitemap Generator' tool that usually hums along quite nicely, but lately, it's been acting like it's got a case of the Mondays, pretty much every day. it's starting to develop a personality, and frankly, we're not loving it.

the main headache is that it's just so darn inconsistent. sometimes, it'll happily chug along, processing even massive sites perfectly. other times, with a seemingly similar site, it'll generate an incomplete sitemap, missing a bunch of pages, or just flat-out give up on larger sites altogether, leaving us with a half-baked XML file. it's like our little website crawler decides 'nah, not today' and just bails. we're talking about pages that are definitely crawlable and not blocked by robots.txt, mind you.

we've gone through the usual suspects, but nothing's screaming 'aha!' at us:

• checked our server logs for any obvious errors during the XML sitemap generation process, and mostly, crickets.
• tweaked crawl depth and speed settings a bunch, thinking it might be a timeout issue or maybe too aggressive. no consistent fix there.
• tested with various website structures and sizes, from small blogs to e-commerce giants, but the behavior is frustratingly inconsistent across them.
• reviewed our internal parsing and validation logic for any recent changes or oversights. everything looks kosher.
• monitored server resources during generation to rule out our own infrastructure bottlenecks, and typically, we're not hitting any limits.

so, we're scratching our heads and have a few theories brewing:

• could it be specific server configurations or response headers from the target websites that our crawler isn't handling gracefully? maybe some weird rate limiting or anti-bot measures we're not correctly interpreting?
• are there common website architectural patterns (e.g., heavy JavaScript, certain redirect chains, single-page application setups) that often trip up sitemap generators, especially when they're trying to build a comprehensive XML sitemap?
• is there a subtle memory leak or race condition lurking in our code that only appears under specific load conditions or after processing a certain number of pages? it's hard to catch those elusive bugs.

we're really looking for insights from anyone who's built or maintained similar large-scale web crawling tools for XML sitemap generation. you know, the kind that have to deal with the wild west of the internet. any debugging tips for these kinds of intermittent issues, common pitfalls you've encountered with your own website crawler, or tools to help diagnose such 'moody' behavior would be super helpful