I can't figure how axii puppet works. I try to cast it 10 times in a row and it triggers one time only. Is it bugged ? Do monsters have some resistance so I need to level it up so It does anything at all ?

Do you know that you have to keep the button pressed until it works?

Yeah I keep it pressed and most of time it just shows normal axii animation. I don't know if it's bugged or it's supposed to be this way.

Yeah, I have noticed it won't work sometimes when I hold down the right trigger to activate it.

It's not just you, alternate Axii is bugged. Even when I hold the button, half the times it casts normal Axii.

I'd advise against using and investing additional ability points into alternate Axii though. The casting time is way too long and makes the ability useless as there's a high chance that you'll get hit, which interrupts the spell.

I completed the game 2-3 months ago but the reason the alt sign doesnt work is another Axii ability. 99.9% certain it was the dialogue convince thingy (pretty sure it the was very first axii ability in the tree with 3 points to max out).

Basically remove that from your build and the alt sign will 100% of the time work.
(aka you'll need to respec)