I have a Nimbus 2 that is a bitch to get the wings in the last 15ml/ half inch even with everything aligned correctly on the marks.

Gary:
Carefully examine the surfaces of the spar butts and the sills that the spar butts slide on during assembly and see where the fiberglass is scuffed and worn from assembly. Put a good automotive wax on all of those surfaces and don't wipe it off. This works pretty well with the problem you describe but needs to be repeated every 6-8 assemblies. try it.
Roy