Check the tapes that have been used in those drives recently. A bad DLT tape can cause leader pickup issues in the drive like what you saw. This was the most common DLT tape drive issue I saw back at HP.

If I remember right, there were three quick things you could examine on the tapes to see if they are OK. Press in the spindle core on the bottom gently. It should spring right back into place. Gently shake the tape, nothing should sound like it is loose. And then open the tape access door and make sure the end of the tape has an unbroken leader on it.