MGS1600GY Track signal lost at stop markers

Hi there

We have problems at the stop markers:

Once a left and right marker is present, the track signal is lost (according to the LEDs).
We use 25 mm tape, the markers are about 15 mm frm the track, and the sensor is about 40 mm above the tape.

Have you encountered this problem before? What can I do about it?

Thank you for the help and best regards,

Oliver