Alright, well I think I found the problem. The threaded metal bolts that stick out of the fin are not the same diameter. First, I noticed that the nylon bolts were both stripped on the forward rod, but both seemed ok on the rear rod. Then I measured the diameter with a calliper, and indeed the forward threaded rod was a bit thinner!

I am going to buy extra bolts anyway, but I think that epoxy may be the only thing that saves me at this point...