There are several possible causes for low UltraCheck calibration RLU values:
Old Luminase
Luminase activity degrades over time. Obtain a new vial of Luminase.
Excessive Heat Exposure
Exposure to temperatures above 30°C accelerates loss of activity. Keep Luminase in the refrigerator when not in use and frozen if not used for a while. If the activity has already dropped to unacceptable levels because of heat exposure, obtain a new bottle. (What is Luminase and How Should I Handle and Store it for ATP Testing?)
Luminometer Issues
The photomultiplier tube in luminometers can become less effective over time. Obtain a Luminometer Standardization Kit and perform a linearity curve check. A biannual linearity check should be completed as regular maintenance
Incorrect Application
Ensure that Luminase is reaching the bottom of the tube and not just the sides because fraction of it will contact the UltraCheck/diluted extract/diluted sample and therefore the resulting RLU will be lower. Hold the assay tube and dropper bottle/pipette tip vertically when adding liquid.