thermostic shower with pump - pump working but still no HW

[MissC]

We have a thermostatic shower which uses a pump on the hot water - the pump is fited straight off the hot water tank and therefore the pressure on all the taps in the house is increased by the pump.

When we turn the shower on there seems to be an airlock and we have to turn the bath or sink tap on and off to get the pump to kick in and send water to the shower.

Recently we have not been able to get the hot water to come through the shower at all (the pump is still working on the taps). This has happened before and we thought that it was because the thermostatic part of the shower was broken, we replaced it and the shower worked again. However, this time when the hot water stopped coming through again we put in a new thermostatic part, which fixed it but the problem occured a week later. We tried taking out the thermostatic part rinsing it and putting it back and it started working. The trouble is it is now stopping working about once a week.

My questions are therefore how can we fix the shower so that we don't need to turn the tap on to make it work and also any ideas on what is causing the shower to stop working so frequently or how we can fix it.

Many thanks