I run into an issue that started the past few upgrades: Mail will just stop downloading new Mail. No rhyme or reason, it just stops. I can have a second compute

I run into an issue that started the past few upgrades: Mail will just stop downloading new Mail. No rhyme or reason, it just stops. I can have a second computer next to it and watch mail come in, but on that Mail, it won't do anything. I have to quit (sometimes force-quit) to get it to start again.