Electric water heating?

I recently moved into a flat which has electric heating and hot water. I have a timer set for the hot water to come on 40 mins before I get up in the morning and nine times out of ten it is fine but on the odd day the water isn't hot at all. The timer is on the same settings for all weekdays so I don't understand why it isn't always hot. I have my own boiler (range tribune he if that helps) and timer so I presume it isn't affected by the other flats. I can't see that there is a thermostat on there either so I cannot change the temperature. Please can someone help?