Strange Problem with PTBurn Server

I experienced something with the Bravo II Disc Publisher system that I don't quite understand.

I have a series of disc image created using a job file with PTBurn Server. I successfully created 10 disc images without any problem. Then I tried to create a set of discs using this set of image files by using a series of PTBurn job files that basically have the exact same content except that each file reference to a particular one of the 10 image files.

The first 6 burn was completed successfully, but the 7th one got stuck and keep rejecting discs. (And all CDR discs are from the barrel I brought!) The strange part was that when I tried to burn the CDR image with the REJECTED discs directly using PrimoDVD 2.1, they all burned successfully!

This is very strange, because I believe that both PTBurn and PrimoDVD use the same Sonic engine. So how come the PTBurn failed but PrimoDVD successed? In additional, how come the burning was successful in the beginning but failed in the middle? I am a bit concern about the reliability of PTBurn Server now.