Calibration Error

We have been calibrating our Sentry 1400 pumps from Tridien Medical for some time. Now every time we calibrate a pump (using the same method we always have) we keep getting Calibration Errors on the last 3 pumps we have calibrated. We have tried redoing the process like the manufacturer suggests when getting an error, but still the calibration error remains. Looking to see if anyone has had this or a similar issue with this product or similar products. It is a Sentry 1400 from Tridien Medical. Low-airLoss, alternating pressure mattress system. Thanks.