I agree with Ari that there must be something wrong in your profile. Of the 10-15 times I have used the kiosk, I have only had one X and that was at ORD and it said on the screen that the fingerprint capture wasn't done right. I went to the nearest counter and was passed right away. A couple of weeks later, I received an email from CBP apologizing for the problem with the fingerprint capture so they have a log if there was a problem. If this has happened to you four times, something is wrong with your profile in the system.