It is not so simple to say.
Are you saying that the timer does not fire at the correct time, or does it not fire at all?
Also, what is the service doing in the meanwhile?
I do not know the exact terminology, but try to visual this:
The thing is, all of these 3 asynchronous runnables (two timers and the service) will run on the same thread. Since timers are message based, the could be queued up while your main thread is doing something else. In that case it will fire quite late, but maybe the message times-out (speculating).