@StefanM, a problem I can have with Argentium is melting it when I don’t want it to, when soldering or fusing. Argentium is easy to melt. A Smith LittleTorch certainly can melt Argentium. Something is not right.
What is holding your 20 grams of silver? Could that be a heat sink, soaking up all the heat? Carve out a cavity in a charcoal block or magnesia soldering block and melt in that.
You say you don’t have enough propane pressure. Why not? If you are using a tank with regulator set the pressure to around 10 psi for the larger tips. A rule I read about with the LittleTorch is use the same PSI as the tip number, or a bit more. As in 7-10 PSI for a #7 tip…
If you are using a disposable tank and assuming it is not depleted, perhaps there is something wrong with the torch regulator. Since you are using a LittleTorch you are using oxygen, right? Can you describe the flame color, shape, and length?
I have not used the multi-jet tip with my LittleTorch. Perhaps that tip needs more gas pressure than the disposable tank regulator will allow? (Assuming you are using a disposable tank…) Can anyone who has used that tip with a disposable tank confirm it will work? Can you try a #7 tip?
This is not the best solution to your problem, but can you make a small ‘cave’ with charcoal blocks or soldering blocks (3 sides and one on top) to contain more of the heat?