I can only think that your problem is that you are not getting the heat into the joint fast enough.A slow heat up will soften the plastic.How do you do when soldering wires to a PCB pad. ?
I don't know what problem you are referring to exactly.. the above was a deliberate destructive test to check how much abuse the joints can take and how well the solder bonds with the wire/header pin. the deformed pins in my 2nd test show that I excerted quite a bit of force while ripping the wires off.. but the remains of solder on the pins shows that it had bonded quite nicely and I am happy with that..
Getting the heat into a joint requires good initial whetting (tinning).Some pressure is helpful.With two loose items this can be a problem to achieve.With thick wire or a lot of copper it take longer to heat the joint.High power iron with a good conductive thick tip is much better than a high temperature iron with a thin tip or low thermal inertia.I am not familiar with your iron.I have a Chinese model which is usable, but i am a definite weller fan.Their tips are well coated , last well, and are still available which some of the cheap ones fail on.YET ANOTHER EDIT.Could you post a you tube video of your soldering technique ?,