Since the interface is small and has to conduct a fair amount of heat, you don't want stuff that dries out quickly. Is your thermal compound high-quality stuff?
Bigger picture: last I knew, there was only one thermal grease that AMD approves if you want to preserve a retail CPU's warranty, and that's the Shin-Etsu 751 stuff IIRC. You can read into that whatever you want to, but the fact that only one thermal grease is AMD-certified seems to underline the position that not all thermal greases are equal.