mixer shower problem?

can somebody please help me i have had this problem for 3 months now and know body has come up with a solution as yet. i installed a bar mixer type shower with a pump on the hot and cold feeds when it is switched on the water does not flow unless the hot tap in the bathroom sink is switched off and on. sometimes this does not cure the problem though,. and i have to loosen the nut on the back of the hot feed to the shower then the pump kicks in and the water flows -or the water flows and the pump kicks in.could it be an air lock or a fault with the thermostat