Inconsistent input monitoring in Logic Pro with Behringer UMC404HD

In Logic Pro 10.6.3 using a Behringer UMc404HD I'm having an annoying problem that is made worse by acting inconsistently.

If I am singing and playing using my vocal and guitar mic I can hear myself fine. But when I press play so I can sing along to the already recorded tracks, I cant hear my input monitoring even though the levels are fine.

This happens even on brand new projects with new audio tracks with no settings. It didn't used to and I cant get to the bottom of it. Worse still, sometimes it lets me sing along fine. Only to not again next time I press play.

Please can someone help me. Thank you.

[Re-Titled by Moderator]