Does the opto register as closed when in switch test and ball is down? Try reseating it to be safe but I bet it's the opto acting up as that is what tells the game the ball is there.

I had the same thing happen on my Luci. It turned out to be a bad connection on the opto board under the playfield. There are several of the same board so you may want to swap it out to confirm it's the problem. You should also take the lower playfield out and clean the optos and everything else if you've had the machine for awhile.

Call Stern. They will help you diagnose and send new parts for free if it's relatively new.

At the very least, take a look under the playfield and check the connectors for that opto. They might just be loose at the opto board where it connects. These things can jostle around during shipping and sometimes just reseating a connector can solve a problem.

The wires in question look like speaker wire with a white connector at the end. if they did fall out, try looping the wire around something before plugging them in to reduce the weight of the wire on the connector.

Go into switch test. Lift playfield just enough to get the ball in the mini Playfield to move out of its resting spot. You should see that switch on the dmd go from lit to not lit. Now lower the pf. Ball will roll back into the resting spot. You should hear a sound and the switch will be lit again showing it as closed. You may be able to test manually with your finger with the pf raised to. Just can't recall how access able that area is right now. You just want to see if it's changing status as the beam is blocked. When it goes from open to closed a sound is made.

If this doesn't help I'll take a video tomorrow testing mine.