If you are not losing pressure, then I would try this first.
I would shut off the water that goes to the shower and remove the center cap on the control knob. Then with a screwdriver, I would remove the screw and that should allow you to adjust the anti-scald device. Turning it to a hotter setting, (Not too hot it is there for a reason).
You will have to try it a few times to get it to where you want it.
If it continues to happen, then I would replace the cartridge.