my triton opal shower varries its temperature,what can i do?

when i turn the shower on it does not always reach the required temperature, its an intermitant fault and seems unconnected to any other factors. One person uses the shower and its fine, next person to use it 5 minutes later and it will not get to the same temperature without turning it up to max, next day it will be ok for 2 or 3 days, then it wont reach temperature on day 4. can anyone help please.