Dollar stores are facing challenges in the modern U.S. economy.Dollar stores companies were some best performing stocks in the S&P 500 during the Great Recession, said Piper Sandler managing director Peter Keith. But over the past couple of years, dollar stores have not seen the same success.Shares of Dollar Tree — which owns its namesake brand and Family Dollar — and Dollar General have both fallen roughly 50% this year.The companies are dealing with a complex mix of both economic and self-inflicted proble ...