I have low cold water pressure in my bathtub ,it works fine for the shower and the rest of the house .It is just low in the tub .What could be the problem ?

If the hot water is fine in the tub it is likely your cold valve or a mixer cartridge. Describe your tub faucet, one handle or two?

If the hot water is fine in the tub it is likely your cold valve or a mixer cartridge. Describe your tub faucet, one handle or two?
It is one knob for both .THANKS

I doubt that your tub spout has a screen or aerator but stick your pinky inside to be sure, if there is one clean it. You have a cartridge mixer valve and there is likely debris inside. Shut off your main and open the sink faucet to blow off pressure. Remove the handle, usually a small screw on the side sometimes hidden by a plastic logo. Then remove the escutcheon plate. Your cartridge may be held in with a clip that needs to be pried off perpendicular to the cartridge or it may be a threaded ring that needs to be unscrewed. Once the cartridge is out inspect the inlet ports on the bottom and clean. Hold a towel over the cartridge opening and have a friend turn on the main for a few seconds and then re-close it for assembly.