How do I increase water pressure to the shower only?

I have great water pressure in my whole house, except the master shower. The pressure is bad from the spout, I have tried different shower heads (and removed the no flow valves) to no avail. I have a recirculating hot water pump that is not functional, but I closed off the system and the water pressure is still no better. The valve has been check and appears fine. The plumber did say that it has a no scald feature and that there are low flow components in the valve that could be removed. I have not tried this yet. Is there an inexpensive way to boost water pressure just to the shower? What could be causing this problem.