What kind of solder tip are you using?
too small and not enough heat is transferred quick enough, too large and the heat can melt the connector.

My basis is the tip width should match the cross section of the wire and pin., The gauge of the said solder tip needs to be equal to or slightly less in diameter overall at the tinning. Thou thru my own trial snd error I am now using a rather wide screw driver type tip on my iron now and recommend a minimum of 70watts. I have used a wide range of tips and whatnot however rarely on anything that required large ones. Back in 1/10 days I didn’t have to use more than 40watts and a 2mm tip.