Also electric showers (unless you have a hi-tech one) only heat by X number of degrees, so as your input water is colder in winter you have to turn it up to get the same output.
Your basic shower has controls for:
Cold, Low, Mid, Hot: These turn on zero, one, two or three elements in the Heater unit.
Numbered dial: This controls the ratio mix between cold water and heated water. So you turn up the dial to get more hot water.
If it is not getting hot enough it is possible you have an failed element or the furring up (hard water areas). The KW rating of the shower is also an important factor as well.