Solutions Sought for Oil Paint Not Sticking

If you're following the fat over lean oil painting rule, is there any reason for oil paint not sticking? This is the problem Terpentijn has raised on the Painting Forum saying: "I paint with oil paint, and I always follow the rule: thick paint over thin paint. I paint on masonite, prepared with gesso, so I have a smooth surface. I found out, to my surprise, that I could easilly remove the last layer of paint, with a palette knife ... It doesn't really stick, it just peels off. It was painted like a month ago. I don't know what I did wrong. Or is it just that the paint hasn't fully dried yet?" Find out what's going wrong...