Plug Power's biggest problem has come back to haunt investors in the hydrogen stock.Shares of Plug Power (PLUG 0.96%) slumped 30% in June, according to data provided by S&P Global Market Intelligence. Investor interest in the hydrogen stock picked up in recent months after the cash-strapped company secured a large conditional loan guarantee to fund its growth plans. A development in June, however, means the funding could run into a roadblock, and that triggered panic selling in Plug Power stock.Why Plug Pow ...