shower?

I have a bath shower mixer. I have not been able to use this since it was instaled.I would say that it is fitted to a low pressure system as both hot and cold supplies run from the storage tank in the loft.
When I pull the diverter knob up the water diverts from the taps to the shower head, however when I let the knob go water returns to the taps and does not remaine with the shower.Ther is no way of twisting the knob to make it stay on.My understanding is that the water pressure should keep the diverter up and then should go off when the knob is pushed down. The force in the spring seems to great to let the water change direction. As a temporary solution I have removed the spring and the knob works by pulling up for shower and pushing back down for bath.Do I have the wrong taps instaled ?

Hugh
May 2010