Shower pump back pressure?

I've recently fitted a shower pump in my parents bathroom. It is a positive pressure single impellor 3bar pump in the hot water supply to bath HW tap and shower mixer v/v. Running a bath it works great, with hot and cold taps open the bath fills quickly. However the shower is not working well. It appears that the thermostatic system opens the cold side before the hot, for safety I assume. But with the cold side open it seems that there is insufficient flow in the hot side to start the pump. If I isolate the cold supply and open the shower v/v the pump will start supplying hot water, then if I open the cold supply isolator the pump remains running and the mixing v/v operates correctly. It seems that the cold supply (at mains pressure) is preventing any flow on the hot side. Any advise on solving this much appreciated.