A couple of events happen on occasion which seem to make training bug,
although they are not really bugs.

1) Your crew's special abilities only take effect while the crew are on board,
alive, and not mutinying. So suppose (taken <completely> at random *cough*)
you have two sailors, both level 4, Bill and Fonz. But Bill is a trainer, so
your max crew rank is 5. If you send Bill off to train to rank 5 (which he can
do), the max training rank for your sailors drops to 4, until he gets back.
And thus you'll get a message that Fonz can't train any more.

Note that when I say 'max rank', I mean the rank they can be trained to.
Losing a trainer will <not> remove ranks from your crew.

2) Along the lines of the above issue, your crew can not return to your ship
until you have summoned it into the game (there's nothing to board). Thus, if
you log in and run to promote a crewmember who you started promoting last
boot, odds are you'll get an 'unavailable' type message. Simply summon your
ship to take care of this issue.

Melemkor