It could be that something else in your house is using the water e.g. washing machine, dishwasher, tap, central heating, etc and this is reducing the flow to your shower and thus causing it to get hot. Check this first. Also water companies sometimes turn the pressure down to conserve water, if you get a plumber ask him if he can check your incoming water pressure. If it is about 1 bar then it is on the bottom limit, maybe he can increase your water pressure by opening the incoming valve slightly. Hopefully these ideas will help. If not then best of luck.
Paul M
January 2009