It can sometimes glitch like that, you need to refresh it in a way or let it update the achivements, it'll catch up fast, just watch if you got the achivement during gameplay, then it's the achivement counter, if it didn't show when you killed the boss it's the games fault
I had a similar problem cause it didn't register few achivements first time then it didn't register 100% for 3 days
It got fixed