Mail app Inbox not syncing between MacBook Air & iMac

To make a long story short, I had to re-create my email account on my server. No problem. I then had to relink things through Mail. This is when all the problems started.

I FINALLY got all my email accounts to both send AND receive (IMAP) in Mail without a problem. That was a whole saga in itself.

But now when I perform an action in one Inbox, this action does not sync in the other Inbox. For example, I check my email Inbox on my MacBook Air, reply to one, archive two, and delete one. When I go to my iMac, the Inbox still shows in the state it was BEFORE I did all the actions on the Air. What gives??? I never had this much trouble having everything sync up, but ever since the email account fiasco, nothing seems to work properly. Help!

FYI: mid-2010 iMac running OSX 10.10.5 & early 2015 MacBook Air running OSX 10.10.5, both with all email accounts set up via IMAP