The first 100 or so Stranger Things had this post issue, I did a short video and explanation on how to fix that damn stupid lock post today if anyone is struggling but you can also get a warranty replacement from your Stern Distributor, mine hadn't arrived yet so I did this mod as I wanted to get the game on location.
This is a short video showing the stages to repair the common problem with these ball lock posts (the plastic keeps popping out).
Firstly, take the post out of the machine and locate the Stern "punch" marks on the plunger as these are the drill points. Take a 3.1mm drill and drill straight through the plunger making sure its straight and the drill will come out the other side in the "punch" marks.
You'll need a slightly bigger drill bit to countersink the hole on one side (to make it easier to tap in the roll pin).
The roll pin, most pinball guys have old plunger and links lying around which will be the perfect size for this, anything like a slingshot link etc will be fine. Then knock the roll pin into the Stranger Things lock post and job done! This sucker won't come out anytime soon!