I am not getting enough hot/cold water from my tub/shower valve, what should I do?

First, start by making sure that the water heater is working properly by testing the temperature of the water at your lavatory faucet.

Next, check your shower valve. Start by removing the handle and loosening the screws on the faceplate. Remove the faceplate, then close the screwdriver stops (These are the two screws located on either side of the valve).

Loosen the cap and sleeve that are under the handle. This will expose the cartridge.
Remove the cartridge and spool. Gently shake the spool side to side. At this point, you should hear a rattle (If you do not hear a rattle, the spool may be stuck).

Soak in a 50-50 solution of vinegar and water for a couple of hours. Flush the lines, then reinstall all the
other components and tests.

Steps to reference:

1. Start by removing the handle (19) on your shower valve by loosening set screw (20) using the appropriate Allen wrench.
2. Using a Phillips screwdriver remove the faceplate. (13)
3. Close the hot and cold stop valves (11) by turning valve in a clockwise direction using a flat screwdriver
4. Remove cap(18) by hand
5. Using plumber's wrench or channel lock pliers to remove sleeve (17) with caution not to let internal parts fall inside the wall.
6. Remove entire cartridge assembly (15+16) by using needle nose pliers, inspect for debris that may be blocking waterway.
7. Shake spool assembly (15), you should hear a rattling sound if spool is moving freely
8. Soak in a 50/50 solution of water and vinegar for a few hours. While soaking spool perform procedure in step #9
9. Make sure to cover area under shower valve with rags to avoid excessive water from dripping inside wall, slowly open hot side stop valve (11) and let water run for a few seconds, close the hot water side and then perform same operation on the opposite side
10. Remove spool assembly from water/vinegar solution rinse with water, test again, if spool is still frozen replace with a new spool assembly. If spool is moving freely within the spool housing go to step #11
11. Replace spool assembly on valve, insert two teeth/tabs into the two corresponding indentations inside the valve.
12. Replace cartridge and sleeve with caution not to overtighten.
13. Open hot and cold stop valves (11)
14. Replace cap tightening by hand only
15. Replace face plate and handle.