Maybe this would help. I wrote this email to CBP after I received an email from them because of a problem I experienced using one of the GE kiosk:

Thank you for inquiring about the difficulty I experienced at the GE kiosk on Feb. 22, 2010 at ORD. I had difficulty with having the correct pressure on my fingers on the scan. Before I had time to correct the problem, the machine proceeded to the next screen and at the end, I received the receipt with the "X" so I had to see the CBP officer at the regular immigration counter. It would have been much better if the machine will allow more time for the fingers to be placed correctly and adjusted or not go to the next screen before the finger scanning could be verified.

I want to congratulate CBP on the Global Entry program as it is tremendously convenient and have enabled me to make tight connections even with a late arriving aircraft.

I would also like to make a suggestion on the question that deals with food. As the question stands, anything edible would have to be declared. I know candy, mints, chocolate, etc., especially if I had purchased them in the U.S. but still have in my possession on re-entry, are allowed but as it is now, I would have to declare "Yes" on the question of food and will automatically get a "X" on my receipt. If the questions are narrowed down to the specific food items that are prohibited, it would save having to go to the regular Immigration counter even if I just have a "Kit Kat" chocolate bar in my possession. I hope you will consider my suggestions. Thank you.

This was the email reply I received:

Thank you for taking the time to provide feedback on the Global Entry program. We apologize for the difficulties that you encountered while using the Global Entry kiosks to enter the United States on February 22, 2010. On this date the kiosk failed to grant you passage, because your fingerprints were not properly recognized; the images captured by the scanner on this date were of poor quality.

We endeavor to improve the quality of the program and are examining alternate means of verifying the identity of program members. In the interim, please ensure that your four fingers are properly placed on the fingerprint scanner when the images are captured. This will allow the scanner to obtain a more accurate image of your fingerprints. We are also making modifications to the system to allow for a second attempt at fingerprint capture, should the first attempt fail.

Please note that if you get an “X” on your Global Entry receipt, you are to proceed to the nearest primary inspections officer (the nearest booth) to be processed. Current CBP policy gives you “head of the line” privileges. You would then be processed by the officer in the booth expeditiously. We have reminded our officers of the proper procedures when processing Global Entry members.

In addition, please note that the question on the declaration screen that deals with food has been recently amended. The question now asks:
“Do you have any fruits, vegetables, plants, insects, meats or products containing meat, dairy products, animals or animal/wildlife products, disease agents, cell cultures, snails, soil; or have you visited a farm/ranch/pasture outside the United States?”

Regards,
XXXXXXXXXX (Name deleted per FT rules)
Program Manager
Office of Field Operations
U. S. Customs and Border Protection