Plug Power has growth potential, but one critical challenge remains.Plug Power (PLUG 12.20%) is a pioneer in the hydrogen sector, and analysts generally think hydrogen fuel has a bright future. Forecasts regarding the industry point to major growth in hydrogen supply and demand over the coming years. Some estimates believe this growth will be sustained for decades to come. Why, then, has Wall Street been so bearish on Plug Power stock over the past few years? There's one glaring culprit.Plug Power stock is ...