Hello Matthijs,

First, identify which sensor is #1 and which is #2.
You can do this by looking at the LEVEL settings page and seeing the (X) mark appear and disappear as each sensor is manually triggered, see State section.

With the Min/Max control function, sensor #1 is the upper and #2 is the lower.

Since you want the optical sensor to be the lower sensor, this should be connected to the 2nd level input on your primary device. Perhaps it is connected to input #1, instead of #2.

We have a video which explains this here: