Hot water trickles (not enough pressure to force out of shower head) while cold water is normal from American Standard single handle bathtub faucet

Remove the shower head and turn on the water. Doe hot water come out of the open shower arm? Let me know, Tom

Removing the shower head made no difference at all the hot water still only trickles from tub faucet and will not come from the shower at all

Is it pressure balanced faucet ? Newer model ?

No I don't believe its pressure balanced. It's maybe 5 years old. Opened and closed valve several times. Every once in a while I would have full pressure, but when it was closed again had only trickle come out of faucet.

If it is Single Handle faucet, the chances are it is pressure balanced unit. Even if it is not, there is a cartridge inside. You have to close water, remove handle and trim, remove cartridge and clean it. But I would strongly suggest that you replace it.

Shut the water off to the valve and open it up. Remove the cartridge and check and clean the inlet ports. While the valve's open place a pan over it and turn the hot water on to flush out the supply. You should have a full sream when you do this. If not let me know. Good luck, Tom