You did nothing wrong. All skins shrink and expand with temperature and moisture changes. I've seen videos of seal skin boats with big loose areas on the bottom's of hulls. Nylon is more heat sensitive than polyester and canvas is worse. I think you made the better choice of fabrics. I always recommend Poly over Nylon. My skin boat is Poly and it loosens a tiny bit on the bottom in cold water.

It's important when doing the skinning that you pull it on to the point of ridiculousness. I use a canvas pliers from a art store. Once pulled tight I staple the fabric to the top of the gunnels - then sew the deck as tight as possible and remove staples as I go. When it's that over-tight, you stand way less of a chance of fabric loosening up noticeably. I have a friend who did it as I described then went back and re-tightened it once more before sewing.

I also used epoxy for a coating which helped - although it's still quite flexible. If you ever attend a skin boat gathering, you won't feel bad. There's lots of loose skins everywhere - it's the nature of the beast. The only thing you can do is make the up-front sewing as tight as possible.

The way Klepper and others like that do it is by having a Hypalon coating which turns the fabric into something different. It's highly toxic, has bad fumes, expensive and hard to find so don't bother.