Hot water slows to a trickle when lever turned to hot. Cold water has pressure. All other faucets in house have pressure

Blocked inside the tap on hot waterside could be some packing in there

These types faucets have a what is called an anti scald valve build into them, it is required by law, it is either defective or out of adjustment, contact a local hardware store for more detailed info.

If I understand correctly, you have single handled shower faucet.
Normally there is a wait for hot water to arrive at faucet.
But now hot water is already at shower faucet.

Single handled faucets have a mixing area inside valve where hot and cold come together before going into sink.
This mixing valve can have crossover where the hot and cold can pass through to each other even when faucet handle is off. When single handle faucets get old, this can happen.
How it happens:
For example another faucet is turned on inside house, and pressure differential between hot and cold causes hot water at shower handle to pass through mixing area and into cold water line.
So if another faucet has been run, then that lets hot flow to faulty single handled shower faucet.
To confirm this is the problem: turn on other faucet and put ear against shower faucet to listen for moving water.
This condition may also signal there is a water leak.
The problem is not harmful, but will draw cold water into water heater each time, and is energy wasteful.
Replace faucet or have plumber repair problem.

it is likely required. many new faucets have what's called a pressure balance valve that helps prevent scalding by lowering the pressure of hot when the cold pressure drops (due to a toilet flush or other water being run). Because you have pressure to only one side, that can cause the valve not to provide the necessary volume.

Can you remove handle , behind handle there should be a dial to move to increase hot or cold supply.

Remember where you start and remove it and replace a 1/8 to 1/4 turn counter clockwise, just put handle on try it and see if it changes and adjust accordingly.

it can be adjust take handle off and remove locking collar first turn water off,

it has a metal or plastic ball with a shaft for the handle it may have spun or a washer may be bad under it,

you can get a replacement at lowes or Home depot to replace it if this is the problem .

adjust ur anti scold the setting is set to low when single handle shower units are shipped they are set in cold mode for safety ur install/instructional paperwork explains such and also advises on how to adjust---remove handle and on shaft will see a blue and red plastic clips that are adjustable slide towards u and move red for more hot water test water continue till comfort level is reached for hot water

check ur anti-scold setting when shipped from factory unit is set cold install instructions always advise about adjusting to suit your comfort--remove handle and using the blue & red adjustment tabs set the heat to your desired comfort

your problem can be fixed by running a loop in the plumbing. If you hired a professional in the first place it would already be done. basically the water pressure isn't enough, i bet your faucet is on the second or third floor, or even just THAT far from the water heater.

sounds like you may have a little debri on hot side stem or kinked hose between valve and spout turn off water to house remove handle pull bonnet and remove stem turn water back on with stem removed see how water flow is be careful turning water on you could make a mess i am assuming the low pressure is in shower only