How can I get my Digistat 3 to come on at set times ?

I had a Digistat 3 wireless installed in August 2006 together with a Worchester Greenstar 30Si combi boiler. In the last couple of weeks the thermostat will not come on or turn off at the programmed times. I have to turn in on manually by increasing the temperature on panel and off my decreasing temperature and then it works. I have replaced the batteries and display is showing as before. My plummer said the boiler is working and it is the thermostat which is the problem. Can you help?