Have you tried just leaving it alone to get on with it's business? Thunderbird will contact the server to get mail when you start it, it will also check again at a regular interval. Clicking to get messages can simply interrupt a hung up download and restart the process.

You might also want to consider disabling any mail scanning as your mail scanner might be nodding off after a couple of emails and leaving the whole thing to just hang. About the only thing other than a defective mail scanner that could cause this is if you have exceeded you bandwidth allocation from your mail provider. But I doubt a few emails would be enough to do that. However streaming movies or backups to cloud drives might.

I tried to do what you said but nothing changes. I deactivated the antivirus plugin, I completely deactivated the antivirus on the PC but nothing. Other accounts download all emails upon first login. The main one of GMAIL downloads only one email every access (every 10 minutes). If I force the download it always downloads one...