QC failure

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analyticalchemist
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QC failure

Good day everyone!
I run a calibration curve with five standards. The following analytes are in the standards Cd, Mn, Se, and Pb. I have a QC  that continues to faiil for Se. The QC contains all the analytes including Hg. The QC fails for Se, but STD2 passes. I began to think that my QC may be getting old. I checked the vacuum pressure and it is not erratic. Se is ran in DRC mode but the other analytes are ran in standard mode. Se has always been ran in DRC mode. The solution been ran is only 2% HNO3, no biologicals. Any suggestions? Thanks again