Do you have more than one instrument that you are able to validate the accuracy and precision of the QC values on? Is it inaccurate / imprecise on one or both (all) instruments? Hopefully you have taken steps to troubleshoot the obvious sources of error with your QC material, itself... ((What kind of QC material do you use? )) Do you have another campus in your system that may be experiencing the same problem?
Which TDMs are you having trouble with? You are aware that some are microparticle and require mixing before being loaded and should be mixed (Disposable pipettes work best) every so often. Mixing well, especially before initial calibration and QC will give better and more consistent results. This way, you are calibrating a consistent mixture instead of the reagent's supernatant. (essentially)
Are you having trouble with other analytes on the CC side of the instrument... even subtle drifts in QC values? We had a problem once and it involved the CC reagent pipettor not accurately pipetting the correct volume of CC reagent up and out to the cuvettes. What it ended up being was the inside of the glass of the CC reagent syringes (the small ones) were dirty, preventing them from being accurate. The othe time, someone had done maintenance and didn't put the syringe base back on correctly, so it leaked and the CC syringe wasn't able to hold the pipetted amount... (Imagine holding your finger on a straw with liquid on it, but instead your finger doesn't form a good seal on the top and it drips out) This problem presents moreso with the TDMs because of the small volume of reagent it uses, versus other analytes. There could also be a small micro bubble on the tip of the CC syringe(s). This can and will affect volumes pipetted by the syringes.
Are you looking at your cal reports the print off? Are they acceptable the first time calibrating or does it take a few times, with some of the duplicates being rejected? How are the blank measurements? Once again if the blanks are failing, you could have reagent issues that stem from improperly cleaned cuvettes to improper amount of reagent in the cuvette. Is the wash tower washing properly? Do you do stream pattern testing on the three washers? Is the cuvette wiper changed regularly and is it seated properly?
Is your wash pump working properly, delivering diluted wash concentrate to the washers? Ours recently malfunctioned, preventing wash soln from getting to the wash tower to clean the cuvettes properly. They were getting cleaned, but not well enough. (think of using water to wash your windows versus Windex, they get clean, but not "clean") When you do your stream pattern test, is the fluid that comes out clear (water only) or is it foamy and hazy (diluted wash conc.)?
Were other things looked into as far as reagent and QC integrity during shipment? Current storage as well as who is prepping the cartridges before loading?