Meta gem not active even though color requirements met

[Jaixx]

I used the 21 int + mana back on spellcast meta gem, but when I added the gem, my total mana did not even increase.
Yes, I meet the gem requirements.

[skullatbash]

Because meta gem ignore gems placed on "additional" socket (the one you add via profession= wrist, gloves, belt).

So, to active meta gem, place the gem to match his color on "standard" socket