The initial version of the 13-Sensor-Flex that shipped with the Shoto HDK has a bug. The middle sensor (middle when counting from either side) does not work.
The problem is that the photodiode has four pins. On the top of the photodiode (pins farthest from the electronics at each sensor site), both pins are cathode so connecting to either one works just fine.
On the bottom side of the photodiode (pins closest to the electronics), the pin on the right hand side (near the two traces) is a no-connect while the pin on the left-hand side is the anode connection. There is a bug in the layout that on the middle sensor and only the middle sensor, the flex layout is missing a connection from the NC pin over to the anode pin to connect the anode of the photodiode back to the TS4231.
To fix the problem, see the picture below. You will need to make an electrical connection between the two pads of the photodiode at the end of the photodiode nearest to the electronics.
The two big arrows in the pic below show the pads that need to be connected.
One way to fix the problem is to solder a small bare wire between the two pads. I used some wire wrap wire and a soldering iron.
First, strip off the insulation completely on a piece of wire that spans the two pads.
Solder the wire between the two pads as indicated by the red line in the pic below.
The Completed Fix
After you complete the fix you should have something like the pic below and probably nicer since I have the soldering skills of a brick.
You will need to perform this fix on both 13-Sensor Flex Assemblies.
Once you've completed the fix you can verify it by running the lighthouse console and confirm that all 26 sensors are showing hits.
Alternative Fixes (untested)
Haven't tried these but you might also be able to use conductive epoxy or a conductive ink pin to make the connection.