My skill don't work!

So… My invisibility skill, normally have 15sec of duration (it’s written on the skill perk) but when I use it I have only 5sec… why please? And it’s like all the other skill/perk doesn’t work anymore like the various damage skill you can have don’t increase my damage.
It only happen online with a friend after some hours of game.

Can somebody help me?
Thanks a lot