Paste is used so that no air pockets are b/t CPU and HSF; some paste transfers heat better than others. HSF has to be firmly on CPU or it would be just like having no HSF at all.
Two things: First, you cannot have a thermal proble after only ten minutes of operation. Two, Even if it takes you a week, both sides have to lock securely. It's like doing the Indy 500 with a partially cut seatbelt.