Actually I think yours is the third or fourth post reporting that Thunderbird only downloads messages at startup, and on further enquiry I've discovered that all of them are Charter customers. One of them pasted data from the error console which said that the server 'mobile.charter.net does not support RFC 5746'. I believe that's about a security vulnerability but I don't have the technical knowledge to understand it fully. Try looking at your error console (maybe clear it first, then try to get mail manually) to see if you get the same message.

I am, indeed, getting the "does not support RFC 5746" error message. But what do I do about it? I still have to restart Thunderbird to see my mail. Charter insists it's not their problem and refuse to talk to me because I use Thunderbird instead of Outlook. I've been looking for answers (have spent over 4 hours searching online so far). Surely I'm not the only person with this problem. One site suggested it's a AV conflict and that I should direct Microsoft Defender to ignore my email program, but I have no idea how to do that (yes, I have researched that, too). Windows 10 requires that you know the extension of the kind of files you want ignored, and I haven't been able to find that info, either. I'm just about to pull the plug on Thunderbird out of desperation. I'd hate to, because I've been a loyal user since about a month after it came out. But I'm not a site developer or an Ubergeek, and I just can't figure this one out. (But thanks for replying. At least now I know I'm not completely mad.)