Could be a couple things. First if the house and pipes are old you may
have galvanized steel pipes, which corrode etc. These after a few
decades can have just a tiny stream of water able to pass thru.
SOlution is to rip them out and go pvc (or copper $$).

Also could be the shower head. Take it off and se ehow much water
comes out. Should be about the same as the faucet.

Also could be a bad valve.

If the house is old I bet on #1.