Known issue with a recent patch changing which level a skill unlocks at. The level a skill is obtained at determines how much xp is necessary to rank it up, higher level requiring more. That means if you respecced just recently you didn't have enough experience for the new rank total needed.
Good thing you get double xp to quickly grind it back up.

It would have been nice if they had told people this in the patch notes, or if level obtained didn't affect the experience to level a skill.
Rather dumb that some skills are slower to level just because it takes you longer to get them already. It's adding insanity to masochism.