There could be various reasons this occurs. see this document's solutions for this error: http://www.symantec.com/docs/TECH37091
and verify you are using Symantec drivers for the tape drive
also run a cleaning job
thank you very much for your information,
anyone knows that the transfer rate running jobs degrades or decreases and how to avoid
you might be facing this known issue. http://www.symantec.com/docs/TECH204119 does restarting the services help?
Run live update to verify all hotfixes and service packs have been installed. Make sure that the remote agent has been pushed out to the remote servers so that everything is patched to the same level