Make sure all chemicals are being stored correctly. All chemicals are labeled with how they should be stored but here is a quick run down
Reagent A - room temp in dark
Reagent B - before reagent C is mixed in - room temp
Reagent C - before mixed with reagent B - room temp
Reagent B +C - put in refrigerator
activation capsules - room temp
VDK caps - room temp
Make sure all samples are treated the same. You should be filtering out yeast and make sure you are using the same filters. Make sure they are taking the same amount of time to filter. If you are centrifuging yeast out, make sure that is always done for the same amount of time between samples. Make sure samples are all the same temperature when you are running the tests. For example let any samples what may have been stored in the fridge warm up to room temp before running the assay.
Make sure your hot bath is at 70C. Use an external thermometer to double check any digital read outs. Make sure that the hot water baths' water line is above the sample's water line.
Only run 1 to 2 tests at a time.
Make sure you are mixing the Reagent A+B+C mixture well before adding it to the caps.
This is the order we add everything when we run a test:
Add Activation Capsule to glass vial
Mix Reagent A+B+C
Measure out sample and set to side
Add Reagent A+B+C to cap.
Pour sample in to glass vial, place cap onto vial immediately on top.
Place into hot bath for 15 minutes.
Try to keep the order and the time it takes to add everything the same.