Soldering drain plug retaining clip

I hate to ask this question but I'm a bit stumped .

I read the post about the retaining clip coming off of the drain plug assembly on the early commandos so I figured I would solder the clip on just as everyone else has. I have soldered several electrical connections with out any issues and think of myself as being pretty good at it. BUT!!! I can not keep the solder from balling up when trying to solder this dang clip on. I'm using map gas to heat the plug and I'm sure its hot enough but I can not get this stuff to flow into the groove. Any ideas? Are there tricks to soldering non copper items.

Maybe I should try some non flux core solder? Just looking for some opinions before I go out and buy anything.

Thanks,

Byrd

Thanks Ludwig - I figured it had something to do with the flux core solder. I'll let you know how it works out.