Why wont my new shower work?

I have had installed a Tavistock kinectic CT50 SKN0706 shower it was working for about a week then no water would come through the shower head and the pump would not start, I thought it was the pump but all checks show the pump is ok, the shower worked again for a short time, it seemed to start better when started on cold then adding the hot, but again it has stopped working, all the pipes have been checked, there is no debris in the pipes or valves, I am at a total loss as to why it is not working can anyone help.