Backup Exec cannot inventory on first try, only able to work on 2nd try each time

Are you using tape labels? If so would recommend just doing a scan rather than an inventory each time.

Beyond that - as it sounds like you are using pre-initialized LTO9 tapes anyway, I would suggest collecting debug logs by running SGMon.exe and comparing the logs collected during the first inventory with those collected during the second (working) inventory.

Probably best to raise this as a ticket with support and let them figure out what the debugs show as the cause.