Pressure in tub, but not shower

My tub has lots of pressure, but as soon as I pull the shower switch, it decreases. I have replaced the head (I thought the old one might be clogged) and the gasket in the diverter switch. I am pretty sure that it is OK, because no water comes from the tub spout when the shower is on. The shower is also louder than the tub. Is there possibly a clogg or something in the line to the shower head, and if so, how do you remove it?

 

Sounds to me like you have one of those watersaving low flow shower heads. Fastest way to check the problem. Take off the shower head. What is the water pressure like coming out of the shower arm. Forceful or a trickle? Trickle means you have a diverter problem. Is the diverter on the tub spout or is there one built into the valve? Some models have a diverter built into the valve that is activated by the tub faucet. This diverter might be stuck and they can be a bear to get to to repair or replace. Some of those diverters can be inserted wrong in the valve. You could have a clog in the diverter area and just about the only way to fix that is to replace the valve.