Why i can't increase my lockpicking?

[Xtenz]

Edit : Fixed, i spammed a chest for maybe 20 - 30 times (yellow lvl) and it finally works, can't find the removing tool so i edited.

I'm still lvl 51 since the begining and i can't increase it. I mean, i did this quest (http://www.wowhead.com/quest=2282/althers-mill) but once at 51, I couldnt up my level on the chest of this quest or on other chest...

It's a little annoying, does someone have an idea?