Replication issues

Couple of issues regarding replication:

We have replicated two database at two different sites and are in the testing phase now.

1) We tried bring the dblinks down on both sides, do some transactions and bring the db up. When we bought the db up,but the data wasnt being pushed to the remote dbs and we had to push it manually. Do you know why this happened? Also, when we checked the jobs, we saw that one of the jobs had been broken. Is there a way to bring the broken job to normal state automatically through any dbms_jobs. I'd really appreciate an example on this , if any.

2) If we check the deferror table, I saw a lot of errors mainly ORA-01403: no data found. I think this has to do with finding null values prior to doing an update. Is there a way to overcome this error?

3) And also if we are setting it up for a failover scenario, how do I set the tnsnames.ora file.

Any help on this topic is very much appreciated.