iSeries VTL and Data Domain Back ups

Hi,

We have the iSeries Back up set up with VTL Data Domain and other applications storage also pointing to the data domin.

When we do the daily back up from the iSeries we are getting media errors.

Is this due to the lack of space for the iSeries?  Sometimes the back up goes through and some times its stuck with Media errors on many volumes. Then we need to re initialize those tapes and we need to use it.

Whats the exact problem here. Please explain  Thanks