mixed bath tap with shower

I have changed a new bath mixed tap with shower attached. the problem is when I am using the shower, the water temperature is not consistent. i.e. suddenly it is cold or hot. but it does not happen on my old mixed tap shower. any idea causes? there is no another's in my house using water when I was using my shower. Thanks

you have probably got a combi boiler and the taps you have dont balance out the pressure at all hence why this happens