Siemens RDJ10RF wireless not turning heating on?

Siemens RDJ10RF wireless thermostat seems to have a mind of its own. I have it set to run the vast majority of the time. Just last night and this morning the heating was working but this evening it isn't. I hadn't so much as touched the thermostat in that time. This problem has been going on for a few weeks.

I have been following the instructions of pressing the reset buttons etc and nothing seems to work. Randomly the thing starts to work sometimes but only after i've spent 1-2 hours trying to fix it. Need help