Levels of units abilities

Levels of units abilities

I have this problem:

I've created new unit ability and set it to have 2 levels, and I gave this skill to a unit.

When I've tried to increase lvl of this ability nothing has happened.

If you want to know I used those triggers:

Event:
Unit is attacked

Action:
Set level of MY ABIELITY to 2

In game, when unit was attacked nothing had happened, and the lvl of skill did not increased to 2

How to make levels of UNIT skills work? Plz help

Problem is sloved,
I`am not sure what I`ve done, but I thing I was using item ability.