Thermostatic shower problem

[Pitchy]

I have just installed a mira cobra thermostatic shower running off a combi.
i keep having problems with the hot water supply being very slow
some days it will work fine and others it will not.
the boiler is on the wall in the kitchen and the shower is in the room next to the kitchen the pipe work runs out of the bottom of the boiler and along the underside of the kitchen base units, there is a tee off to the kitchen sink that works fine then the tee off to the shower that works sometimes then there is a tee off for the bath that works fine and then the sink in the bathroom at the end of the line that works fine.
The only difference i can see between all the outlets is that the taps are below the combi and the shower head is above. if i run the bath tap and lower the shower head to below the bath tap and then turn the bath tap off the shower will work fine again.
do i need to fit a checkvalve or something any help would be greatly received.
thanks

 

[ChrisR]

WHat combi is it?

 

[Pitchy]

its a baxi 100 he i think

 

[Pitchy]

i have also removed the shower from the wall and the water flow out of the hot pipe is just a trickle.
but i can fill a bath full of very hot water in just a few mins from the bath tap.