First, check the camera calibration by pressing the asterisk key 11 times
Ideal values are
R = 0
C = 0
E = 100
Normal (acceptable) values are
R +/-2
C +/-2
E 100 ~ 120
If you're out of Normal, then you can open the clock and clean the inside of the angled glass as well and use compressed air (duster can, not shop hose) to clean the camera. See this video to remove the camera and access the inside of the angled glass
http://sundialtime.com/support/HandPunch_Internal_Cleaning.MPG
If that doesn't work, you can try adjusting the reject threshold.
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Using WHVB (usually found in c:\RT3) or WH32 (Download: http://www.sundialtime.com/support/WinHand32.exe) you can
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Once connected, open Reader >> Setup Data…
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Click "Get" to read the current values.
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Adjust verification “Reject Threshold” to the desired value. Default is 100 but could be set as high as 125. Higher is more forgiving, i.e., will accept more often.
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Click "Set" to save.