Thermostatic shower - lost of hot water pressure?

Our thermostatic shower, which is has a shower pump fitted, has been working fine up until this week. The pump is activating as normal, however there is a huge reduction of the water pressure and no hot water at all, when the shower is set at a warmer temperature. However when we move the shower to a cold temperature the presure picks up and the water gets more cold. any ideas what could be causing this and how we can fix it?