If the cloth has no resin on it (no dry flaky feel to it) then try putting some more epoxy on it and maybe some clear packaging tape to hold that area down as it cures.
If has cured epoxy on it and it just lifted off the surface, you might have to just sand that section out and cut a small fiberglass patch of the same size and epoxy it in place. For such a small localized area, I don't think you need to overlap the new cloth onto the existing fiberglass. If you do remove the small patch that wouldn't behave, sand the surface where it lifted off from. You might have gotten a bit of grease or something on that surface.
But see if you can do what Jimmy said first before sanding that area out..