18kt Medium solder is not flowing

Hi James,

I have been working with 14kt red gold fabricated ring soldering 18kt
yellow gold rails on either side. I have had trouble with the 18kt
medium solder I have been using, not good flowing of the solder.
Using welding silver solder paste flux. I will also be using 14kt
white center with 18kt rails. Any suggestions? A particular company
that excels in quality of their solder?

Thanks and best regards,
Richard Hart G.G.
Denver, Co.