We have the same problem as Suzanne back in November. Our bathroom sink gets hot water O.K., but the bath does not. We replaced the cartridge, MIXET Shower Valve MOdel# MS5AT PK per instructions. STill very little hot water, it trickles out, just like before, except that hot & cold is reversed. We have checked twice the installation & part #. When we access back side of the valve, the pipe is getting hot, but no pressure out the faucet. The sink is ok.
If the hot and cold are reversed then inside of the faucet you either have the cartridge or the valve inside backwards. Try rotating it 180 degrees. Before you do that, remove all of the parts from inside of the shower valve (shut off the water valves first). If what remains is behind the wall, make a little plastic or tin chute to guide the water back into the tub area. Now turn on the hot water valve, slowly at first, and keep turning. Watch the flow of water. If it still trickles then your line is clogged and you will need to replace all or part of it. If the water comes at full force then the problem is in your valve assembly. This is known in the trades as "process of elimination". Good Luck!!!!!