Do you physically feel your hs heating up during loads? I can on mine but mostly from the rear half of the thing. Try to feel down on the pipes by the cpu during loads, they should get quite warm. if not then it's not seated properly. I used artic silver 5. Also try to feel where the air is flowing after it goes through the heat sink, it should be able to escape quickly. I use a square of toilet paper sometimes to get a visual of airflow, or suction. It works great on 80mm fans.
I know people say less is more on the thermal paste, however I used a fairly generous amount, in a combo of the smear with finger in bag, and small dot in center. No doubt I will get criticized for this method.