when you use a planar the curve(s) MUST all have the same translate value....in other words...the curves have to be flat (and you can have only 1 curve)...also you may get an error in the operation..if that is the case..it's usually because a curve is not closed (edit curves...open/close curve)....or because it is not self intersecting....a lot of times a loft will do fine....post a pic if you are still having probs so we can better help you