Originally Posted by ballengerb1
Sounds to me that the inlet to the mixing valve is getting clogged up with crude. If the tub doesn't get good pressure the shower won't either. And you did say that this is a new problem so I'm going with crude. Shut off your main, open the faucet at the sink to blow off pressure, remove the handle and escutcheon (plate thing) likely you will see a ring shaped plate with two screws holding inthe cartridge, remove and inspect the inlet ports on the back. If there is no plate or screws tell us what you see. Speedball probably can identify your valve.