Shower pump problem

[barnet boy]

My brother has a loft extension with an en-suite. The water tank is also located in the loft space on a raised platform.
The shower is fed via a shower pump in the airing cupboard on the floor below. The old shower worked OK but was very tatty. I have replaced this shower and located the new one at the same height on the wall. However, now the pump does not kick in when the shower is switched on.
If, however, when the shower valve is open, a cold water tap anywhere else in thje house is turned on then the pump kicks in and the shower is OK.

Any suggestions?

[barnet boy]

I just realized that I've probably introduced negative head onto the pump. I'll need to go and check. Would this explain the problem?