Perhaps these rubber bands are old or very used and allowing too much strectch in them. Or maybe the wing is smaller than most trainer wings and need shorter rubber bands. But using 14 rubber bands on a 4 lb plane the wing should even move. I suspect the rubber bands arnt tight enough.

Modifing the fus and wings to use 4 screws could work, but a pain to take off.

Perhaps you are ready to start flying a more advance flyer. There is alot of things a trainer can do, but usually they dont do well at them.