ISTAT Chem8 correlation to Siemens Atellica BUN not matching

Hello.  We recently got new Siemen Atellicas in our lab and I'm finding that the BUN on the CHEM8 cartridge will not correlate within the 2mg/dl or 9% difference we used for our old instrument.  All other cartridges and analytes are comparing well.  The controls and calibration on the Atellicas look fine.  The controls and cal/ver looks fine on my Istats.  Has anyone else seen this with their instruments?  I contacted both Siemens and Abbott, and they did not have any information for me.  Any suggestions or input if you have seen this with your Istat/Atellica comparisons?  Thanks.