Same thing happens to my boys when they wear bumkins. It's one of the reasons why I don't like bumkins much. They're just a little too plastic, IMO. I don't understand the argument that the bit of mesh at the back means that the cover is breathable. Plus, after a couple of messy poops-out-the-mesh, I began to wonder why anyone would ever have any part of a diaper cover be mesh.
That said, I still do use them occassionally. Like, in dire emergencies, a.k.a. when all other covers are in the wash.
Sorry, no help to you!