It's all about heat transfer. Old solder, especially lead free solder, won't be very receptive to getting heated by the iron. As others have suggested, add some leaded solder to the joint, this will make it much easier to melt. The solder should also contain flux in it, which helps to clean up the surface of the joint so it takes heat more easily.
I don't know how much experience you have with soldering, but if you don't have much, then make sure you understand how to manage the iron. The tip should always be kept clean and tinned. Depending how powerful the iron is, you might also need to wait for it occasionally to build up some heat before you apply it to a joint. As soon as the iron touches the joint, all the heat is lost from the tip and it needs time to recover.
Most people find that chisel style tips transfer heat better than conical tips. A larger tip will have more thermal mass which allows it to melt joints more easily.
If that's an unregulated 40W station then I think you can get by with it, but if you do this very often you might want to get a temperature controlled station of higher wattage. I wouldn't recommend a high wattage unit without it being regulated though.