Triton Delta

The things to check first are.

1 That the main valve coming into your house is fully open.

2 Then check that the valve on the line to the shower is fully open.

If these valves are not fully open then there may not be enough water
getting to your shower to open / close the safety device that allows power through to the elements

Also as it is a Triton have a look at the outlet TCO

The outlet Thermal Cut Out this is a black disk with two wires going to it and it is mounted on the out let pipe (The bit between the heating tank and the hose)
Examine the connections closely if you see any discoloration / signs of overheating then the TCO needs replaced.

The water coming into the shower is much cooler in the winter time therefore the shower can’t heat it so well.

If it is unacceptably cool then you may have a faulty element.