Soldering Stranded Copper Wire
I have been running into a lot of that funny wire lately.  It looks just like any other wire I have used for years - but it won't wet right.  Upon closer inspection, I see some of it was copper coated steel wire ( magnet attracts it ).  Some of it was copper plated aluminum ( wonder how they did that ), and other was plain old stranded aluminum wire ( it was a dollar-store USB cable I bought to hack into for a source of five volts ).  

It looked great until I tried to solder it.  At first it would seem to wet, then solder just rolled off it.  Not even GOOT flux worked.  What happened is the solder dissolved the thin layer of copper off.  Must have been electroplated on.  When I took it to the lab and saw what was going on under the microscope, it became obvious what the problem was.

I guess with today's technology where most everything is crimped, aluminum and steel wire is a lot cheaper than copper wire and works good enough to avoid a customer return. 

Google sent me here while I was seeing if this situation existed.  The evidence left here shows that is indeed what is going on.  I leave my experience for others also trying to figure out they were losing their mind because they can't do something they have done for years.