1/16 isn't much but is it the entire wing seat that has the gap or just the trailing edge of the wing? Have you sanded it a bit before covering? Maybe sanded a little too much there?

If the other wing seats perfectly and this one does not you could consider adding a little balsa sheeting there, which you can get in 1/16 thinkness I'm sure and that would eliminate the gaps. Also - check the length of the aluminum tube - I built the kit as per instuctions but my tube was about 1 1/2" too long so I had to cut it down a bit to get it to the ~15" it was supposed to be.