I'm trying to visualize this. You've sewn the ribbon which is woven to the jersey which is stretchy?

That's going to go badly. If the ribbon is the tiniest bit shorter, the jersey will be puckered. Also, the jersey won't stretch if it needs to. If the fabric stretched the tiniest bit while you were working it, (it sounds like that's what happened) the ribbon is longer than it needs to be and when the fabric relaxes back, it will wrinkle the ribbon. Even when fastening ribbon to a solidly constructed bridal gown, we usually just tack it loosely at the side seams, or make a little loop crocheted out of thread to hold the ribbon belt in place. Stitching it all around is a recipe for disaster.

I'm sorry, that's not much help.