I've successfully silver-soldered using a 140-watt soldering pistol. In fairness, the wire was only a little over 1/16" in diameter (.078).

What is the diameter of your silver solder wire? Mine is only about 1/16" diameter. I had a friend who gave me some silver solder once, about 1/8" in diameter. I couldn't use it cuz I couldn't get something that large up to temperature. Is your flux clean?

I believe you when you say that you cleaned and fluxed the parts properly. But I've had problems like that in the past, too.

Is it possible that after you cleaned the parts you may have wiped them down with a contaminated rag or paper towel? Something that might have had some oil (or whatever) on it?? Or perhaps you touched it with your hand or whatever (body oil)?

If you were using a 'lightweight' soldering iron, it's possible that the wire absorbed the heat away from the joint. in other words, you couldn't apply the heat fast enough.

Here's a little tip. Melt a little blob of solder onto the tip of your soldering iron. Then apply that blob to the joint. That little blob is NOT going to be part of your solder joint. It's only function is to transfer the heat.

When you can feed the solder from the opposite side of the iron and the metal is hot enough to meit the solder on its own, you'll have a great joint.

As far as using electricians solder with rosin flux, it's not as strong as silver solder.