Possible Cause #1
Unclean Mouthpiece
Solution:
Try sterilizing the mouthpiece in hot water or use a new mouthpiece
Possible Cause #2
Blowing too hard
Solution:
Blow a long and steady breath into the unit until it stops beeping or till you hear/feel a double click/beep.
Possible Cause #3
Unit due for calibration
Solution:
Book your unit in for calibration at Caltrace