There are 3 main causes of low pressure in a shower. The first is the showerhead: Turn on both the hot and the cold water. If they're both coming out slow, the problem is likely your showerhead. Remove it and clean out the screen to fix the pressure. Alternatively, the valve might be bad. If you have plenty of cold pressure and low hot pressure, the problem may be inside the valve itself. That means there's debris caught inside the handle where you demand the water. If neither of those seem to be the cause, you may have build-up in your pipe that's restricting the water flow.