Yup, you need greater localized heat. An oxy setup will probably do
I used to have a problem balling up the axle wires on sterling omega
clips. Most of the time if the ball was successful the clip lost
tension. I started using 14KW wire. Problem completely solved. Sure,
it cost 47 cents more per pair, what did I care? Now though I just
use the $15K laser to secure sterling wires, what do I care?
Tongue in cheek if no one noticed. But my point is you need to
invest in what gets the job done.