SylvanLMP
Has any of your “plumbers” checked for an integral stop that maybe slightly closed or blocked?
A lot of shower bodies come with these isolation valves (globe type) that offers lots of restriction
You could also look for a tile stop (Shut off valve) that you can isolate this showerbody, then disassemble then flush out to make sure you have full flow going to it.
When reassembling this showerbody use new OEM rebuilding parts.
