When soldering, be sure to apply a separate flux. A rosin core flux doesn't act until the solder melts and this is too late for this kind of joint. Be sure the flux gets into the joint, even to the point of flowing out the other side. When properly fluxed, a joint will suck the solder into it so strongly that you can apply the solder on one side and it will flow all the way to the other side. In fact, that is a good clue that the solder has completely filled the joint.