My friend has the Pro version and having the same problem (got his like 2 weeks ago, probably made in the same batch). I called Stern yesterday on his behalf and they just offered to send me some new part. I forget what he called it, but it sounded like it was a newly designed part, as opposed to just a replacement part, but I could have misunderstood and maybe there was just a bad part that was put on the machine originally.
As a side note, my friend mentioned was that when the playfield is up in the air for maintenance, the drop target works flawlessly when manually activated.
I'll try to do some proper documentation here once the part gets here (unless someone beats me to it).