My filters only work when I set them up and intermittently after that.
Before I spend another minute on this - are filters supposed to work on IMAP?

They should work. You may want to see if your IMAP server offers sieve, which in the case of IMAP is IMO better (server side filtering vs client side).
Can you make an example of a non working filter?

Weird ... when I posted a rule here, I noticed all my rules had actually lost what they should do with the filtere message ...
No wonder they didn't work

Well, better to have a simple solution that no solution. If the issue is fixed for you, can you mark this topic as solved (use the checkmark icon)? Thanks!