you also have to line the chip up in a certain way, usually the chip will have a small triangle in the corner of it, you must put that triangle towards the lever
Just a side note regarding thermal paste. There are some that are glues, and overtime, even non-adhesive ones can become quite sticky. I have an old P4 in my closet I can't get the heatsink off of! I'm afraid something will break if I try much harder!