Sounds like you may have bad cartridges or downlevel microcode.

Do you have the 2X feature on your drives? There is a green 2X sticker on
the back and the internals of the drive are green instead of blue.  I do not
know that the drive will stop you from mounting a K tape in a J only capable
drive.  There was about 8 months that E1As were shipped without the K
capability.

If K tapes get jostled around a lot the labeling area on the tape gets
damaged.  We had this happen to our tapes early in the K days, but not
recently.  I can tell you, if you are having trouble labling these tapes now
you are going to have hard failures later.

Get your CE involved.