My Mira shower cools down?

My new Mira Excel which runs off a new oil fired combi boiler cools down after around 5 minutes. So far the plumber has changed the cartridge which made no difference and cleaned out cement like debris from the hot water valve leading to the shower resulting in a little improvement. He has also altered the temperature limits within the shower but it still cools down. We have very good water pressure generally. Does anyone have any thoughts on what the problem might be.
Many thanks