I am the one that suggested checking the shape of the leading edge of the main wing. If the two sides of the wing are not identical the plane will have snap problems. I would concentrate on the main wing with this snap problem. Check these items:

Is one side of the wing twisted in relationship to the other

Make sure the leading edge of the main wing is the same on both halves.

Try and keep this plane in the 11 to 12 pound range for the best flight characteristics.