This might happen if sample-types other than plasma or serum have been analyzed.
Our quality control system is based on the plate median value for the Incubation control 2 and the Detection control which are added into all wells of the plate along with 2 other internal controls. The measured values of these internal controls will differ depending on the sample matrix they are added to. The buffers for IPC and negative controls are made to mimic serum and plasma to make the values for the internal controls in the same range as in the samples. If the samples run are not plasma/serum, the control samples will therefore differ from the plate median and might fail the quality control.