Dear Old Uncle Milty,
You shouldn’t have trouble soldering the two metals together. Which
parts are made of which metal? It sounds like you may not be
getting the larger part hot enough to properly melt and flow the
solder. You can heat the living daylights (well, up to a point…)
out of nickel-silver because it melts at a much higher temperature
than either silver or the solder. Judy Bjorkman