Stock alarm clock temperamental..

[matt504hrs]

Had no problems until recently.. It's set to go off at the same time every day but the last two weeks on Monday morning it just doesn't go off and the clock icon in the status bar vanishes. When I go to alarm clock it is still ticked to go off but doesn't return to the status bar unless I un-tick and re-tick!
Lucky I have a back up alarm on my bedside clock otherwise I would be screwed!
Any ideas what has done this and how to rectify it?
Thanks

 

[SUroot]

Do you use a task killer?

 

[matt504hrs]

No task killer!