After more study I'm starting to think tightening the wing nut was only part of the problem I was having with the electric fence being weak.

What I missed until looking at yesterdays picture was the washer placement. It seems clear now that more surface area touching the opposite wire means more charge.

Another mistake may have been wrapping too many strands, which I think created gaps in the connection. My new method is a simple bend that wraps around the bolt without making a complete loop.