Boilers?

Hi, We've just moved into a new flat and I am trying to work out how the timer on our vaillant boiler works. The clock makes a ticking news, but never seems to go round, and as a consequence the boiler doesn't come on if I set it to the timer mode. It otherwise works fine. Is this a fault with the clock/timer, or am I just being dense? The timer is in clock-shape, with one painted on arrow and one arrow that moves round. There is also a small marker on the outside of the clock dial. You turn the dial round to adjust the times, and push the little widget thingies up and down depending on what time you want to set.
Thanks!