I've also had the same problem. Solved it by mixing another batch of epoxy and brushing it over the stuff that didn't cure. It worked.
Next time, you might consider using Balsa Rite. I painted a piece of balsa with it and put it in a baby food jar of 10 percent nitro and closed the lid. A week later, I took the balsa out and wiped it off with a paper towl and broke it. It snapped. It was just as dry as ever.