Maybe it is improperly mounted.
I say maybe I go to fast sometimes!
It's not perfect.
I remember going onto the GSP and my tag didn't read so I backed up a bit to try for a 2nd time, but nothing doing. So I took a quarter and threw it in the bin, but still nothing, tried a 2nd quarter and still nothing. After that off I went...
Those machines have a mind of their own.
That wouldn't be good should they charge you a surcharge for every time the machine decides to act up.