Hi,
The cron you show there (backupmng) just checks the BackupsScheduled table in the psa database and if certain conditions are met it then runs the backup, so its fine for you not to see the individual domains mentioned in the cron.
We don't experience this issue on any of our servers where the scheduled backups work fine with the exception of when Plesk decides to do them at completely the wrong time (restores are another matter but I'm inclined to think thats us causing that).
Could it be that this is being caused by other backups failing to finish and the system thinking that your 10 backups are all running in some way? (You could try raising the limit, or check /var/lib/psa/dumps/tmp for directories or check the BackupsRunning table, to be honest I'm unsure exactly how Plesk decides how many backups are currently running).
Paul.