I had to tech a buddies machine with this problem. I put the game in single switch test and watched the matrix as I triggered each switch. I noticed one switch flickering when I tested the other switches by coincidence. In single switch you select each switch and when trigger is makes a sound. However, other switches will not make a sound if triggered. I just happen to look up at the DMD and saw another switch flicker. I knew I found the culprit. It was the drop target switch diode was bad. Drop target switches take more abuse because the up and down motion of the mechanism. The fact that your drop target isn't resetting leads me to believe this is the issue.

Yes, the switch is under the playfield. When the target drops, it triggers it. Keep in mind, IIRC, the target drops during the start of a game to load and stage the balls under the cannons. However, it should pop up once the balls are staged.

Read this thread. Tons of good information.

https://pinside.com/pinball/forum/topic/100-sttng-fix-me-challenge-experienced-techs-only

That's it. Pull off the subway and clean them all. The subway looks intimidating but its not that difficult to remove. Clean and lightly re grease all those diverters.

Also for reference

Pinguy is the expert. He got mine running.