Folks I’m having the very same issue and even if I still couldn’t get it to work, I think I could figure out how this works.
Read the points below and check the pictures to get the full understanding.
1. The plastic “thing” that seems to be only a limiter to the arm is not only that, but you’ll see that there’s a very tiny end of that plastic part that stands out a little bit, exactly on the end that meets the shiny silver plate.
2. The shiny silver plate is a micro switch that is soldered to the main board, and the switch itself is a little black part in the inner side of that plate into the body.
3. The way it wasn’t engineered and it’s supposed to work is: If the plastic “thing” on the arm is P E R F E C T L Y in the place and angle as it comes out of the factory, when you fully extend the arm, the little edgy end of that plastic part will match the micro switch actuator (black piece), sending the signal that it is extended.
In my case I had the two back arms over extended a little bit causing a little bending to this plastic thing, not much but the necessary to make it not perfectly latch the plastic little edge to the micro switch actuator.
This is very frustrating. I’d hope for a more intelligent and less sensitive solution for that part, considering the size of the arm and how easy it is to bend that plastic part that plays a crucial role both on structure, by limiting the over extension of the arm and the sensor.