OK, are your surfaces totally clean and dry? It sounds like you have
some kind of contaminant in there somewhere. What you might have to do
is heat the area you want to solder, and then brush some flux on it.
This will clean the surfaces. As a Plumber, I can tell you that it can
sometimes be quirky. You meantioned you've tinned the parts, are they
shiny and all silver colored? Before you started heating to solder them
together, did you add fresh solder to the joint? It's not complicated
really, but I always found that there were a few things that had to
work together, and that could sometimes make it complicated.

Good luck, and let us know how it works out for you.