programmer?

we have just had a sime comby boiler installed and i have tried and tried to set the wall programmer for the radiators to come on timed in eg. 8.00am to 9.00am to go off and 12.00 to 1300 to come on and go off and then 1700pm to 1900pm to come and go off, but they keep coming on at different times to what i have set them any explanations?