Winter is already blowing cold for retailers. Just this week, J.C. Penney (JCPN.Q) reported a 26% decline in same-store sales as Kohl's (KSS 2.82%) warned that steep discounting could take a big bite out of Q4 results. Yet neither fared so poorly as Groupon (GRPN 2.07%), which fell 29% after reporting a meager 3% gain in European sales during the third quarter. Troubling? Sure, but declining cash flow is far more worrisome. Find out why in the following video.