Are they of the correct density (e.g. HCART)?
Are they in the library? (sorry for that!)
Is the scratch pool configured as such?
http://seer.entsupport.symantec.com/docs/294593.htm
Has the tape physically expired?
http://seer.entsupport.symantec.com/docs/316585.htm
Has the media reached its maximum mount limit (if set)?
***EDIT***
Nearly forgot this (!) :
In-depth Troubleshooting Guide for Exit Status Code 96 in NetBackup Server (tm) / NetBackup Enterprise Server (tm) 6.0
http://seer.entsupport.symantec.com/docs/278555.htm