Some really cheap cables use unrefined recycled copper. Since many recycled copper comes from scrap wall sockets or power strips, they have high phosphorous and beryllium content, which makes them hard to wet by solder.
Solution: buy high normal quality cables with either low recycled content or refined recycled content. You will not see crappy copper in Longwell or Volex cables.
Edit: s/high quality/normal quality/g
If they add the phosphorous as some kind of deoxidiser or something, that might make sense.
But its hard to believe there would be enough beryllium in there to prevent it from soldering. unless they dumped a metric ton of relay contacts in with the 10 tons of copper. but i don't think that would be enough.. also if someone did have a metric ton of relay contacts, it would be recycled for the beryllium and silver, not dumped in a copper melt.
but if someone were to melt down 100 tons of power strips.. there would be enough brass in with the copper cord to raise the resistance to unreasonable levels.. however, still no beryllium
I have run across some 10 ohm, 6 foot long, iec power cords. not making that up. the servers they came with only pulled about 100 watts, so they would have survived, and their resistance was discovered after finding one of them was open circuit. the wires were not magnetic, had a color somewhere between brass and light brown..