I doubt that it is heating it up enough to remelt your solder joint.
I think your problem may be a dry joint / the joint not wetting properly to begin with.
Soldering is a less is more type of thing for the best part, that goes for the heat and amount of solder you use.
60/40 resin core solder is ok, but eutectic solder is a hell of a lot better, it will eliminate problems caused by having a plastic point like dry joints caused by movement.
To make sure you get a good join, clean both surfaces with isopropyl alcohol, then with your eutectic solder tin both surfaces ie. apply a small amount to make them shiny silver something like this:
Then clean them again with the alcohol so that all of the flux (brown crap, I think you guys call it rosin) has been removed. then hold the two surfaces together, apply the iron and a little more solder, clean the joint again and your done.
It is also worth mentioning that unless your tinning the iron, you should never apply solder to the iron. Apply the iron to the joint and the solder to the joint, let the heat conduct through the joint to melt the solder, when you apply the solder directly to the iron you burn off most of the flux (which is what is making the join wet properly) and may not get enough heat into the items your soldering for them to join properly with whatever flux is left.