My Free XML Sitemap Generator Is Suddenly Forgetting Pages – Is It Confused About Website Indexing?

Alright, so my 'Free XML Sitemap Generator' has decided to develop a personality, and it's being a bit forgetful lately. It's suddenly not including all the pages from my website when generating the sitemap, acting like some URLs just don't exist, which is definitely impacting proper website indexing. Anyone seen this kind of selective memory before with their sitemap tools, or have any quick fixes for why it might be missing pages crucial for search engine visibility? Help a brother out please...