With large wire, you need a lot of heat. I do tin the plug but not the wire, it may prevent the wire going into the plug. Try to heat the wire and plug at the same time.
If you’re using EC5, you will solder the connecter before you attach the housing. Be careful not to get any solder on the outside of the plug. If you’re using another connector, attach an extra connecter, plug it in to the one you are soldering. This will prevent damage, holding the plugs in place if the housing melts a little.