CUPS uses three log files, by default located in /var/log/cups:
-rw-r--r-- 1 root root 70426 Feb 21 08:56 access_log
-rw-r--r-- 1 root root 25436 Feb 21 08:56 error_log
-rw-r--r-- 1 root root 424 Feb 20 16:40 page_log
access_log: Seems to log every HTTP and IPP request, look here if incoming print jobs don't print. See Advanced Features down below for configuring CUPS to accept remote print jobs.
error_log: General CUPS log file.
page_log: CUPS seems to log all print jobs to this file.
If you are still having problems then you might consider writing to a mailing list or newsgroup.