Two key uncertainties make the prospect of buying it today riskier than is desirable.DexCom (DXCM -0.95%) is not a stock that investors should be rushing to sell. It has a few different opportunities to continue pursuing to generate growth, it has plenty of recurring revenue, and the valuation of its shares is reasonable. But you still shouldn't buy it right now.The odds are good that it'll continue to be a decent performer moving forward. Still, it's facing two key issues that need to be navigated successf ...