It is probably a faulty element in the heating tank (there are two, three in 10kw showers) you can check them with a multi meter you should have a reading of around 12>18 ohms.

If you don’t have a multimeter we sell them at £10 each with a guide on how to use them for checking and testing the various parts of your shower.

Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses, letting you know much power you have left in batteries

(so you know if they are dud or not) and lots of other jobs, in short they are well worth the £10.