Mail not downloading emails

Mac Mail is not downloading files from the next unless I close it with Activity Monitor and open it back up. When I come back, it download them at start up time only, and not ever again during the session. Also, Mail is not downloading emails quickly when it does come back.

Maybe one problem is that I have 38GB out of 1TB. Could be? I don't know.

I have tried a few things like connection check/Rebuild/etc... None of them work.

Please let me know if you have any other ideas.

Thanks!

Willie