When it's happened to me it's either been not enough/slick enough lather, or I had it angled to the point where the razor head behind the blade was coming in contact with my smooth face and sticking.
Its happened to me when I've overtightened the blade retention thingy on a Schick Krona / Gillette Aristocrat. Blade gap gets way too narrow and there is no edge left to cut so it just drags across face. Happened to me this AM.