Detector Chip does not work if detector 10 is not connected
The detector IC cannot detect/process IR hits if detector 10 is not connected.
The corresponding pin at the IC is permanently low and therefore blocks the processing (inputs are inverted and ORed) The most likely cause is that the programming switch detects the floating input line from the detector as low and therefore forces the data line low (so the pullup in the detector IC) does not help.
Usually we will play with all detectors, so shouldn't be a big deal.