Sounds like you need to replace you diverter spout or if you have a valve that switches the water from the spout to the shower head you need to fix or replace the valve . Another thing you could do is put a diverter spout on and when you want to take a shower you pull up on the knob on the spout and presto you have a full stream of water comming from the shower head. : when i switch the knob for the shower to turn on, water comes out of the top shower and also out of the bottom spigot causing loss of pressure .