OK, so I had a short cage SRAM X9 mech and swapped my rear cassette to a 11-34 from an 11-32 – the 11-32 worked fine. Now the mech gets caught behind the 34 tooth gear and won't come back down. So, I switch to a medium cage mech to see if it helps and that does the same. This is all stuff that came off another bike and worked no problem, so what gives? Chain too short? Any ideas? I've tried that adjuster that swings the top jockey wheel away from the cassette and that doesn't help.

*sighs*

chain tension is the issue

b tension on the mech surely – needs winding out more to pull the mech backwards / away from the casette

Winding in pushes the mech away

I've tried that adjuster that swings the top jockey wheel away from the cassette and that doesn't help.

Sounds like the b-screw to me
you wound it right in, to push the mech away from the cassette?
it's possibnle you've found the limit of the mech or, as rr says, chain could be too tight to let the mech pull down (how does it look?). Waht setup – 1×9, or are you on the granny ring ?
You could maybe try a longer b-screw in place of the existing one (was wondering about this for 36t on a normal shimano mech)

It's 1×9. I suspect too short a chain. If the chain was longer the arm of the mech would be dropped further away from the cassette. I'll give that a try tomorrow.

The b-adjuster didn't let the arm of the mech drop enough at either extreme of its adjustment.