CG is definitely it but also you need to see what your engine incidence is and what your elevon reflex is. As a very general rule your elevons should have about 1/32" or 1/16" up reflex. Your engine should be inline with the wing with no up or downthrust. Wings are highly sensitive to CG changes so once you get it flying, spend an hour or so dialing it in. With every flight things will change. I like my wings on the ragged edge and use airfoils that are not so violent when it comes to snapping out.

Good luck and post pics. Let us know how things turn out.