I've an old problem that seems to re- appear

A while back I glued a set of Ply doublers on the balsa fuze sides for a Gensis 35,
I used slow curing 24 hr epoxy, applied only a thin amount to both the doublers & the
balsa, weighted the assy's down with some alloy plates on a perfectly flat surface,
after they cured I inspected the job, nice flat assy............thats until a couple of weeks
later when I had another look at them and the balsa had "curled" along the length of
the doublers .....why does this happen, moisture in the workshop?, should I have
"angled" the ply - grain to 30o?.. as in this case the grain on both the doublers and
fuze sides ran the same way.

Any tips or hints greatly appreciated

Right on the mark guy's

I took the sides indoors last week , placed them on a flat surface, and
now they have returned nice & flat
need to really take care on the storage & climate of all woods in future