My American Standard R120 shower valve has excellent pressure on the cold side, no pressure on the hot side.

SOURCE: Not enough hot water

Some higher end models do have an adjustment that is behind the face plate. You would have to remove the handle and the screws in the faceplate to expose them. If you have adjustments they will be located on the brass valve body facing you. You will need a flashlight and a flathead screwdriver to make the adjustment. If your model is not equipped with adjustments you could have a faulty valve stem. The stem is comprised of a rubber band like gasket that separates port holes into which the waters flow. There could be some piece of trash blocking the port not allowing adequate water from the hot side into the valve.Some valves stems are also able to be installed upside down causing the same issue as the ports do not rotate fully to allow adequate supply. However you say that you have adequate pressure on the hot side but not warm enough. There is another possibility also based on the way the shower supply was plumbed but there is no need to go into this unless this does not resolve your issue. I hope this is of assistance and thanks for using Fix Ya. Please let me know if I can be of further assistance.

SOURCE: moen single handle valve tub/shower

The Moen Posi-Temp Valve is a pressure balanced valve and when not used for long periods of time they can stick causing only cold or hot water to pass through the valve.
A New Posi-temp cartridges can be purchased at most plumbing supply houses or big box outlets such as Lowes or Home Depot.
As long as you had cold water before, a new cartridge will solve the problem. In the future it would be wise to occassionally operate the valve at least a couple of times a month in order to keep the pressure assembly from becoming lodged in position. Just follow the instructions included with the new cartridge on the proper removal of the old cartridge and the installation of the new cartridge. a little silicone grease inside of the valve before installing the new cartridge will also help.
Be sure that there is no particles of the old cartridge left in the valve.
sanray

SOURCE: no cold water from Moen 2253 tub/shower valve,

Here is a quick check to see if the cartridge may be at fault allowing the cold and hot to mix. Turn off the hot water supply, with hot water supply off turn faucet on. Turn to the hot and cold sides, If you have only cold water then your faucet cartridge is at fault or the cartridge housing is worn. Shut all supply valves and remove cartridge and inspect housing for signs of wear. If wear is visible a new shower valve is needes. If none visible replace cartridge. I have included a link of a PDF file an exploded parts list. I hope this helps in solving your problem. Please rate if the answer helps in any way. Good Luck and have a great week. Jeff