I have three hunters, one orc level 12 and 2 night elves level 9 and 5, that cannot learn skinning and leather working.
Do you have any additional details?
Have you learned other professions?
Are you able to find skinning and leatherworking trainers, and have the necessary silver to train, but are unable?
Clearly, you have stated all your characters are level 5 or above, so that doesn’t appear to be the issue.
You need to start from Journeyman trainers, not from artisan or expert trainers.
Yeah, I’m going with wrong trainer, or already have professions.
Go to the closest major city and ask a guard for the trainer.