Soldering 4MM Plugs

flux really is the key to a good solder joint. I gets the solder flowing and takes the crap out of the joint.

At what temp are you soldering? You should be able to touch the iron to the outside of the connector and get the solder flowing/melted by touching the solder to the connector. If your iron isn't getting the connector hot enough to do this within a few seconds, your iron isn't getting hot enough. Either that or it's having trouble holding its temp when you touch it to the connector.