Can't solder the new switch anymore. Solder doesn't stick to it

This is the 3rd switch that fails over the last year. Yet I had a really hard time desoldering it. I even fear I had broken part of the circuit.

During desoldering A had a lot of solder left, I had to move the switch wire back and forth so it broke apart from the remaining solder and then once free melted the remaining solder from the other side. Also during desoldering out of B a very tiny “copper” sheet came out, I’m not really sure, is that part of the internal wire?

When placing the new switch, I could solder as usual one of the contacts©. However, the one I had troubles desoldering on the first place A&B doesn’t work anymore. The solder doesn’t melt on the switch wire. It only melts on my soldering iron. I don’t understand what is going on. I heat up the switch wire yet I can’t get the solder to melt on it.

I have almost no experience soldering, I just soldered/replaced 2 switches before. They worked out fine.
Thank you in advance for enlightening me.