1. Soldering is similar to welding, a lot of folks can melt solder and get metal to stick together, some can make a nice solder joint, but only a few can make consistent, excellent solder connections. Good soldering comes with technique and experience. As some have mentioned, you have to use the right materials (flux, solder, wires/metal), but the solder iron and tip are also very important. The techniques change depending on the materials and the material mass.
2. Temperature: the iron must have sufficient wattage and temperature regulation to quickly heat up the connection, but not over-heat the connection (which torches and unregulated solder irons tend to do). To compensate for an unregulated heat, you have to guess when the temperature is just right. If you guess wrong, you end up with a cold solder joint or a grainy, over cooked connection. Either way, you have a weaker connection.
3. I have soldered landing gear with a Weller soldering gun and got good results, but you have to be patient and the technique has to be right-on. For landing gear especially, when you solder smaller wire to larger wire, try to position the solder tip to allow it to transfer more heat to the larger wire. You may have to move the tip to adjust the wire's temperature. Practice on some extra wire before you solder your gear.
(I'd better get back to work now!)