Apple Mail Issue: One gmail account folder won't save 'sent' email. Now it is only a black box instead of folder.

I've been trying to resolve this for weeks: In the example below, the black square is a gmail address that will not keep copies of mail I have sent. It is just a black square.. Since it does not save sent mail - those emails do not show up in any search. I have to go to gmail itself to search for sent mail from that specific account.

I've tried removing/adding that gmail account several time, changing settings to match my other gmail accounts. One day this just started happening and it's really a drag because I can't search emails sent from this account. It does not even store one when I test sending things.