Below are 2 illustration for my problem.

After selecting the top and left edges of my SubD (figure 1), applying fullcrease, I can't seem to get the effect I want (figure 2).

Nothing happens.

I have a lot of other similar areas and it's a 50-50 chance that I get a sharp edge.

I can't find out what seems to be a problem.

(The figures are part of a car that I'm trying to model. )

Thanks.