Yes. I fought with mine for 2 weeks before finally calling Grace- and they solved the problem in a 3 minute phone call! Doh!
(I have a Juki and a Grace GMQ frame- so may NOT be the same answer for you.)
The issue with mine was that one of the sensors was tilted too far down, too close to the rail. (on mine, the sensor wheel touches the carriage wheel, not the rail) She told me to lift it up. I may not be explaining it accurately, but I kept thinking that the sensors had to be low, and it was the opposite.
If this doesn't solve it, because it's not clear- call Grace Company. They are amazing and once they explain it, you will have no more trouble.
Happy New Year
Mags