[Noob question] Gem not levelling up?

Hi guys, first of all, sorry for my english.
This is my first character in poe, so probably my question is silly.
I'm a ranged shadow, and my problem is, even if i met the requirements, my Explosive Arrow gem is still level 12. The " + " icon to level up the skill is not showing up.
I made a screenshot
http://i57.tinypic.com/k47zhg.png
I'm lvl 61 with 223 dexterity, others gems are levelling fine (most of them are already level 14-16).
What am i doing wrong? (i'm pretty sure it's just me missing something rather than a bug XD)
Thx

Click the "+" button at the bottom of your inventory. Problem solved.

Wow i'm blind XD Thx!!