BE 12.5 sometimes needs second tape, sometimes not

Assuming that the tapes are showing up in BE as "media full", but the bytes used is lower than you expect, then I suspect that this is another case of a clogged tape head.  Maybe it is intermittently clogging for now, which would explain why it works good on some nights.

This is a tough thing to diagnose as no errors are reported to BE.

This technote should help get you started, but you will probably have to work with the hardware vendor.

http://www.symantec.com/docs/TECH193200

Tapes report full before their Native Capacity is reached

Experience tells me this is a drive issue, as Larry Fine has pointed out in his initial post. You can clean the heads of the drive unitl the cows come home but that won't gurantee to free tape heads which are clogged.

When heads are clogged, HP LTO drives will still back up the data but in a degraded mode...such as lower capacity, slower performance, or lower/no hardware compression. Eventually the drive will reach it's internal thresholds and toss a Tape Alert(s) and that's when you could find yourself unable to backup or restore your data.

Nip this in the bud now and have that drive replaced before it's too late.

BTW: Here's a post from an NBU customer back in 2008 who had the same problem and resolved it when the drive was replaced:

https://www-secure.symantec.com/connect/forums/lto-3-tape-capacity