Sometimes the plating on the metal used to make the contacts of those output sockets, doesn't let the solder "take" or wet it properly and I've found that if you rub the surface of the solder tags of the socket with some sandpaper (say 360 or maybe 400 grit) the solder will wet it better and the solder joint is much improved.
When it comes to preparing a piece of wire for soldering to a solder tag, there are four steps that will help to improve the solder joint:
1, Cut the wire to length.
2, Strip-off the plastic insulation from the end of the wire.
3, Twist the individual strands of the wire together so that they make or form a single strand.
4, Tin the bare wire end with solder so it looks shiny.
When you solder the wire to the solder tag first make sure the joint is mechanically strong since solder is soft, secondly you need to use the tip of the soldering iron to heat the bare wire and the solder, then apply the solder to the joint while the soldering iron tip is in contact with the bare wire and solder tag, a good solder joint should only take about 4 or 5 seconds to make, also, never use the soldering iron tip to transfer the solder to the tip because the rosin-flux core in the solder will have gone up in smoke, the rosin-flux core of the solder is there to clean the oxides off the surface of the metals to be soldered so that the solder properly wets the metals.