First five assassinates each gave me 1%. But after 80% in assassinate it just got stuck, 6 assassinates in a row without any single +1%. I even gained 2 levels because i thought it might matter.

Log:
SNIP

All last six times are like it.

Assassination improvement varies on a number of things.

You have a CHANCE to improve, it isn't always going to improve.
You can increase your chances.

But if you're assassinating a lot of easy targets you aren't improving your chances, so you can't just pick and assassinate a bunch of easy/low targets and expect consistent improvement.