Lost "sent" mailbox

Somehow I've lost the "sent" mail box in one gmail account (IMAP).

One account of 3. In the other 2 accounts the "sent" boxes are still there. And mail from those accounts shows up in them.

When I go the web mail page for the account in question - there is a "sent mail" box there with recent messages.

I've looked in OS's Library and found an account named AosIMAP-gene4318 that has a sent mail folder but does not show recent activity (I just sent email from that account)

There's another account named IMAP-gene4318@imap.gmail.com that does not have a sent mail box in it.

The AosIMAP-gene4318 and MAP-gene4318@imapg.gmail.com are a mystery to me. i.e. I don't know the significance or how they are different.

How do I get the missing box back?

Running: OS X 10.10.5 (Yosemite ) Mail ver. 8.2

TIA

Gene Williams