How can I fix the heating on my Baxi Combi Instant 105e?

I have a Baxi Combi 105e. The hot water and heating is on but the heating is not working. When I turn the temp up on the digistat the flame comes on and the heating comes on for about 5 minutes and then goes off. It does not continue to heat the radiators to the desired temp. The pressure on the boiler is fine and the hot water is fine. The wireless drayton digistat box says that the heating is on and that the radio signal is being received. Can you help? Thanks