As already said, this is the most likely cause. I am also on Win platform and I have to put the plugins in the same folder in order for them to work.
According to Python Plugin Development for Pcbnew,
~/.kicad_plugins (note it is
~/.kicad_plugins and not
~/kicad_plugins) is the correct locations, but you might try putting them in
Looking at Kicad source code lines 175-194 there are a few possible locations. If you try them all and report back the results it would be phenomenal.
Bear in mind if you started KiCad, and only then created folder and copied plugin, the plugin will not be visible until you restart KiCad (not only pcbnew, but whole KiCad).
But if someone using Linux chimes in, you should take his advice more seriously.
Have you heard of KICAD_PATH environment variable? Can we use it as a pointer to user folder with plugins, the same way we are using other enviroment variables?