Make sure your wing halves slide together with absolutely no gap in the center joint. You may have to sand the wing jointer a bit, and maybe even the end ribs.

All of the ones we've seen that had the center joint come apart have had insufficient gluing in the center. Use epoxy that has at least a 30-minute working time. Longer is better.

Load the pockets for the spar, and really get a good coat all over the mating surfaces of the center joint, especially at the leading edge, trailing edge, and upper and lower spar caps. You cannot use too much glue on the faces of the center ribs.

When you push the two wing halves together, a lot of glue will ooze out. Wipe it up with a paper towel or disposeable rag. Any residue left can be cleaned up with rubbing alcohol. Put some masking tape over the joint to hold it all together while the epoxy cures.