The problem could be a couple of things...leaded solder has a lower melting point than lead free, and the flux in the 2 solder types is probably different as well, so that's probably not helping either.
I'd always suggest a pure joint (
) is more reliable!!! Do you have a solder sucker or any de-soldering wick to clean the joints up with?
It might be too late, but if you'd like some leaded solder I've still got some and always have it with me at club. I also have a solder sucker, but no wick right now, that can be cheaply picked up off ebay though.