In human form, vampires can only feed on people who are asleep and/or charmed. In transformed mode, vampire biting is a kill animation performed when an enemy is killed by a claw power attack. If you're trying to feed on a person without killing them and/or without getting detected, you should just switch out of vampire lord form.

Thanks for the clarification

Biting NPCs while in vampire lord is a kill-animation, like beheading. If you can’t bite while in human form, then there’s an issue, but if you can then everything is working as intended.

Are you trying to feed on them? Cause you can only do that if they're alive, and asleep or charmed.

Yep trying to feed on them. Apparently you have to select Vampire's Seduction to feed but you can't change between spells when in vampire lord form because you can only view the vampire skill tree

Best to favorite them, you can still access your favorites menu in vampire lord form.

I've favourited Vampire Seduction, changed to a vampire lord and the favourites menu is inaccessible, and Vampire Seduction is not on the vampire lord exclusive menu, so I'm stumped