The sensor is not centered on the fingerboard
Glueing the sensor to the fingerboard can be a delicate procedure. We highly recommend to use some water to make the fingerboard
The glue doesn’t stick to the fingerboard
The adhesive layer used to manufacture the sensors is quite strong. If it does not stick properly, it might be, that the fingerboard has a particular repellent coating.
Consider visiting your luthier to ask for his advice.