请问这个代码为什么Buy和SellShort会失败

Series<Numeric> MAvalue; Series<Numeric> MAvalue2;MAvalue = AverageFC(close, 60); MAvalue2 = AverageFC(close, 20); PlotNumeric("MA1", MAvalue); PlotNumeric("MA2", MAvalue2); if( cross(close, MAvalue)) { PlotString("buyicon", "B"); bool bpkSuccess = Buy(2, close); if(bpkSuccess == false) Commentary("BPK failed"); } if( cross(MAvalue, close)) { PlotString("sellicon", "S"); bool spkSuccess = SellShort(2, close); if(spkSuccess == false) Commentary("SPK failed"); }