Mixer shower is only warm when hot tap on

[Phunk]

We have a mixer shower which has recently had some issues going warm unless another hot tap is on in the house.

Any ideas on this one? Hot water is coming from a 5 year old combi.

[AndStilliRise]

Water pressure not high enough, so the system can not detect anything open from the shower at a guess. Check your pressure it should be between 1 and 2 bars.