Good Morning.
The problem started after tha migration to a newer version of oracle - migration to oracle 9i. Before the migration jobs that were scheduled in crontabs were running ok. but after the migration they are not running.
I understand that is not easy to find out what the problem is. But the question is: ?The problem has something to do with the database, or the operating system, how can i be parsing thes problem?.
Thanks a lot for your guide.
pick one job and run it manually ( I assume that it's a mix of sql and shell script)
since the only thing that has changed is the database level, try to run the sql part from command line
might be a path issue? check your paths in the cron job.
-S
It is Oracle envirnment settings. Crontab is UNIX untility and run from server. If oracle envirnment was not set properly. Crontab couldn't kick off.
We had a similiar kind of problem, well not for Oracle but for Informix.
As pointed out earlier, majority of blame is to be attributed for the environment settings.
To check that,
make sure all the oracle bin and oradir are pointing to the right directory
export all the oracle env in a shell and run a specific script at a time from command line and redirect the errors (2) to a log, that would be easier to debug
if that is running fine, run the same job through cron
it would be less troublesome if you are running and testing everything on a test box which has the same configuration rather than on a prod node ( luckily 
we had two such nodes )
most probably these should solve your problems!