Retail analysts warn grocery store players such as Supervalu (SVU -3.6%), Safeway (SWY -2.5%), and Kroger (KR +0.4%) are having trouble putting together a quarter of both sales growth and margin upside as increased competition from other formats - notably Whole Foods Market and Costco - takes its toll. While the trio of grocery store stocks lost money for investors this year with the trend in place, WFM and COST have ripped large gains.