I've never had great luck getting any kind of plastic cord to melt the way I want when using a lighter/flame 😂 it's just not a precise way to apply heat, either the location or how much heat. If you have a wood burning wand, you might have better luck with that.
I have a wood burning wand that I have yet to use, I told myself I COULD burn wood and that was enough reason to buy it 😂 You're welcome! Also wear a mask, burning plastic fumes are no joke lol