Do you mean this flux? http://www.cooksongold.com/Jewellery...rcode-997-6418
I use it for both silver and gold and all types of solders (although on silver 99% I use hard), it works well.
Hard silver solder from Cookson flows well even on dirty surfaces, but I had problems with medium from Cookson, don't know why. Scratching the solder strip with file helped. But still I strongly prefer hard.
I have handheld butane torch only (max flame) and I don't have problem to heat rings in range 7-10g. I don't do heavier, because I find anything above 1.5mm thickness highly uncomfortable. I think yours could be approx. 10-13g? It should be doable. I made also some bracelets in 20g range with butane torch, but I used gas cooker in kitchen as small help
I would think you are not getting it hot enough. Or the ultrasonic. Hard to tell. Can you test the solder and your process on something less heavy?