Make sure you removed the protective strips before trying to seal the sensor.
Use the palm of the hand to stretch the sensor over the curvature of the fingerboard.
If the upper layer was not sealed correctly it can be because:
the ground strip was not placed back in place before sealing
the upper layer was not correctly closed, possibly resulting in an un-even surface.
If this is the case, you need to remove the sensor and start the procedure again with a new layer.
The adhesive layers used to seal the sensor are very fragile.