Achievement not unlocking the FIRST time I meet the requirements?

[Seiryu]

We all know of the issues involving the DLC achievements and the issue with crashing right when an achievement would unlock, but I just encountered something I haven't seen mentioned.

I just reached level 20 on a Mage and the achievement for doing so did not unlock. This is the first time that I have done this, and my system did not crash when I reached 20. I'll admit that I used the leveling glitch to get to 20 (I prefer melee classes in RPGs and just wanted to get the Mage achievement out of the way), but I had not seen anyone mention this causing the achievement to glitch and there has not been a patch recently so I don't see what would cause it to happen.

Has anyone else had any issues with the achievements for reaching level 20, either by using the glitch or actually leveling up legitimately?

[when_vegans_attack]

When you hit level 20 you must actually press :back: and level up your character - simply hitting the level is not enough. You must upgrade talents, abilities, etc.

[Seiryu]

When you hit level 20 you must actually press :back: and level up your character - simply hitting the level is not enough. You must upgrade talents, abilities, etc.
Ah, and I would guess that because I didn't actually "level up" until after reaching level 21 that is the problem. I think I made a save when I was around 14, so I will reload that and try to stop right at 20 to see if that makes it work.

EDIT: Yep, that was the problem, thanks!
As a note to everyone else so you don't encounter this, as when_vegans_attack said, you must press :back: while you are level 20 and allocate your points. If you reach level 21 without doing this then you will have missed the achievement.

Hopefully while patching all of the other issue they will patch this also and make it so that the achievement unlocks as soon as you hit level 20 so that it will not be missable.