Yu,
There are two things I can think of that might be happening:
- the timer can not/will not fire if there is no (knowlege) session present and linked to the process instance.
- in some cases, if the node before the timer node (or after) is not correct and fails, that exception is sometimes not shown and the process can silently die.
Would you mind posting of the bpmn of the three nodes: the one before the timer, the timer node and the one after the timer?
Thanks,
Marco