There are 2 possibilities I can think of.Are you using 100% cotton fabric?? If so did you wash it first??If not and you are using steam when you apply the interfacing,you could be shrinking the fabric,causing the interfacing to bubble/wrinkle.

I always wash all my cottons before I sew them,better to be safe than sorry.
Kathi

I'm sure someone else will be along eventually to add their comments.Just may be a few days as ths forum isn't very busy.

That is a know problem with peltex. I've seen a newer interfacing that is not supposed to do that, but I haven't tried it yet:
http://www.lazygirldesigns.com/products/item/Stiff_Stuff/307/p2c56

I've also used By Annie's Soft and Stable. It's kind of like the foam you find in a Vera Bradley bag,

Kathi, i dont wash my fabric for these projects, but the pieces look perfect after applying the interfacing, even after pressing/steaming. it is only at the end, after i turn them out, that they become crinkled, so i think it's caused from the interfacing retaining creases from being so bunched up.

clt3, i hadnt seen these, thank you for the ideas. how do you like the Soft and Stable? any drawbacks? the video shows a nicely supported bag body. i've only used fusible's for my bags, does the sew-in stay in place? would it be stiff enough for purse flaps, and could I still apply magnetic snaps? Thanks!

Yes, the Soft and Stable stays in place very well. It works well on purse flaps. I would probably still reinforce the area for the magnetic snap with a small piece of peltex or other heavy interfacing. Many local quilt shops or sewing machine stores carry it.