This is a common failure mode.

I believe what happens is ultimately due to the nylon bolt mounts at the TE's. The plywood lugs the blind nut come up through are recessed well below the bottom of the TE which allows for the wing to slide to the rear a bit in flight.

When the wing slides back (its only bearing against about 1/8" balsa sheet on the top of the fuselage, which may be pretty soft), even a little, two bad things happen. The bolts now have a bending load placed on them in addition to tension, and flight loads start to pry on the dowels in the front with MUCH greater force than if they were a snug flush tight fit in the plywood former's holes. The lite-ply former with the holes is also marginal IMO. Those holes in the soft ply eventually get a bit whupped out and become sloppy.

The nylon bolt blind nut issue can be mitigated by gluing spacers with a hole in them to the top of the lug block. The holes in the ply former and the dowels can be hardened up with some CA to slow down their deterioration.