Thremostatic Bath Shower Mixer Cold Only!?

Hi, I have just fitted the above the manufacturer is Reef if that helps. However I can't get any hotwater out of it, there is hotwater on that pipe as I can draw it off on the basin but no matter what I do I can't get it through the Bath taps. If I take the bath taps off then hot water will come through as normal. The cold water is from the mains and the hot is from the cylinder as it's a gravity system. Any help would be much appreciated.

Regards,

Jamie..