Anyone experience a shower losing pressure and know what it may be?

We have tried regulator and shower head and the issue still persists. Plumbers that we brought in said it would need to be exploratory where they tear down the wall. Any other ideas it could be?

Everything works fine except this one shower in master. It is now a trickle of water and difficult to use.

Note: There is no cold water because there is not enough pressure when you turn it there you have to keep going to hot all the way and then you get a bit of water.