The most probable reasons would be:
- a technical issue
- a sample that gives a readout very close to the cutoff for being flagged
We typically see less samples that do not pass the quality control in our diluted panels (Cardiometabolic, CVD III, Development and Metabolism), since many matrix effects that might interfere with the analysis are diluted as well. There is also a statistical chance for a sample to fail the quality control, just as there is a chance for false positive results.