EW -

Solder flows to fluxed metal that is at the proper heat. If your solder is only going one way, it is likely that you are heating the two sides unevenly. I am however having some difficulty imagining how you could be doing this, given the size of the pieces. But - take it for what it's worth.

As for the parts moving, a wooden jig, with clamps may solve the problem.

Good luck

Dan Odenweller