SQL & PL/SQL

Hi,
I've designed a pl/sql procedure which, apparently, works fine. I've planned a job with the dbms_scheduler to run it everyday at the same time. My problem is that sometimes the job finish without problems (it takes two hours and half to finish) and sometimes it seems as if the job enters is infinite loop.

Is there any way (or tool) of knowing why it fails?

Thanks in advance.
Regards.