Only hot water delivering to shower

Check the Shower valve for residue or limescale

What type of showers are they? For example are they a digital aqualisa? More than likely it’s a problem with the cartridge as showers usually stay in same position so the cold side probably has a limescale build up.

It sounds as if u may have a pressure problem. Check if your unvented cylinder is piped correctly. After the mains incoming, just before it fills the cylinder there should be a combination valve, this valve has a number of parts but you are looking to see if the balanced cold has been piped, you may need to refer to google depnding on ur knowledge etc. The balanced cold is there to make sure that the pressure on mixers (taps and showers etc) are always equal. It sounds like this could be the issue. Without the make etc and a picuture of the installation it is difficult to say though. Also is it pumped, tank fed (from loft cws) or all mauns etc? U really need a qualified peraon to find out for sure. Good luck.

It sounds like the cartridge but for the two to go at the same time is very odd. I would certainly call the manufacturer of the showers as they should still be under guarantee. Depending on the brand, some manufacturers have their own engineers who will come out and look at the fault.

Most probability is a non return valve letting by or thermostatic mixing valve failed hot water letting through the cold lane. need to rectify the spot where the water is letting by