Investing.com - While many Wall Street firms remain bullish on Walmart (NYSE:WMT), two have turned cautious following the retailer's breakout year in 2017.Oppenheimer and Zack's both recently downgraded the stock, citing concerns about the retail giant's ecommerce business.Oppenheimer said it appears the forces behind Walmart's large online sales gains are waning. The company's ecommerce sales growth in the fourth quarter was half that of the previous quarter.Zack's cited costs associated with ecommerce investments and stiff competition. Though Walmart's made great strides in its online business, Amazon (NASDAQ:AMZN)'s lead is still growing. Walmart stock gained more than 30% in 2017, outperforming the S&P 500. So far in 2018, it is down sharply compared to the broader market.