Mixer taps with shower hose/head connection

[Joef]

The hot and cold from the taps run fine but only the hot runs through the shower head.
What would I need to replace to get the cold running back through the shower head?

 

[Welder]

Apologies but the UKPF crystal ball is in for its annual service

We need more info. Pics are good.

 

[Burger]

The hot and cold from the taps run fine but only the hot runs through the shower head.
What would I need to replace to get the cold running back through the shower head?

Is it gravity fed? Mains fed, had any work done recently and as Dave said, pics would be good.