Mixer shower

I've just moved into my new house and I'm having a few problems with the mixer shower.

The temperature seems impossible to get right. It's either boiling hot or freezing cold and appears to only accept water from one of the two taps.

The couple who lived here before me had obviously used it for quite a long time, so what am I doing wrong? Anyone got any ideas?

A couple of possibilities spring to mind...

Is the shower fed with gravity hot water and mains cold water? That imbalance in the supply pressures would make the temperature setting
very sensitive. Just the smallest opening of the cold supply could overwhelm the hot supply, and possibly feed backwards up the hot supply pipe.

It is possible to live with this if you get used to setting the mixer valve with a micrometer

If it is a thermostatic valve, are the hot and cold feeds the right way round? If they are reversed, you end up with a positive feedback loop. The thermostat opens the 'hot' valve, but gets cold water through it, so it opens the 'hot' valve further looking for hot water until the cold water is fully open, or vice versa

Cant think of a way that one could be used without straightening out the pipework though.

Thanks for your help! I think you're right and I'm going to try and get it sorted out before my baby arrives.