Providing more info may help. Are you saying you can see messages online, but not in Thunderbird?

Yes I can see them at aol.com, but not in thunderbird.

OK, puzzling for sure. I assume you have checked the synchronization&storage section of the account to ensure it's set to keep all sync'ed. Also, check the 'advanced' tab to ensure folders are set for downloading. My other suggestion is to click File>offline and select an option to attempt to download the entire inbox. Otherwise, I do not know why they aren't being viewed.

Yes synchronization and storage is all set up and folders are set for downloading in the advanced tab. I tried File>Offline> download/Sync now, but it didn't seem to do anything. Ugh so annoying. But I noticed it's not only thunderbird not showing these messages. They are also not showing up on my iphone and Outlook as well. I tried getting help from AOL but they just said they do not support third party tools...

Well, at least you confirmed it is not a Thunderbird issue. Somewhere, there is a setting in AOL that is keeping those messages from being accessed.

Yeah, I just wish I could figure out where, there's about 6 months of missing messages, so weird.

Is there a limit on the number of messages in each folder? I noticed my inbox says I have 10000 messages on thunderbird but there are 16495 on AOL.