The hydrogen stock has dropped sharply from its October highs on valid grounds.Shares of Plug Power (PLUG 1.26%) slumped 25.3% in November, according to data provided by S&P Global Market Intelligence. Even after the big drop, the hydrogen stock is up over 120% in just six months, as of this writing.Is this an opportunity to buy Plug Power stock, or should you remain on the sidelines? Plug Power is facing setbacksPlug Power has been the frontrunner in green hydrogen, and its contract book has grown steadily ...