One wire prong not balling up

I am working on a Argentium silver wire reversible bezel (a project
featured in an issue of Art Jewelry magazine). The ends of 6 of the
"prongs" are balled up. However one wire prong refuses to ball up on
the end. I have used the ultrasonic and pickle for cleaning and
deoxidizing, and have even filed the metal to make sure it wasn’t
covered with solder. It still will not form a ball. I am sure that a
number of you out there know exactly what is going wrong. Please help
me out. Thanks!

Two things come to mind

  1. perhaps the material somehow got mixed up and the piece that
    won’t ball up is something else

  2. a heat sink problem. If this one wire is too close to whatever is
    touching it, it may suck the heat right out. Silver is a very good
    conductor of heat.

What exactly happens when you apply the heat? nothing at all?
discoloration? slight deformation?