Some of that wire is either galvanized or has some type of coating. You gotta sand/steel wool through that to get a good joint. Sounds like you:

Might not be heating the joint enough.
Don't have enough flux.
The wire is dirty.

Clean all the metal parts with sandpaper or steel wool.

Construct a solid mechanical joint.

Apply the flux liberally. I use the blue paste type.

Heat the joint. If the landing gear wire is larger than 5/32", I'd use a propane torch. Even the big Weller 140w gun won't do the job on large wire. Wave the tip of the flame over the joint, heating it evenly. When it's hot enough, touching the solder to the wire will cause it to melt and flow into the joint. Never heat the solder itself. When the crevices of the joint are all filled, pull the solder and heat away.

Let the joint cool naturally. Don't move it, and don't apply water or air to cool it faster.

Good soldering comes with practice. Keep at it.