I had a similar issue on a machine I had worked on for a customer.  By hand everything seemed to check out OK, however a finger does not make a good representation of a balls motion.

I'm not sure if it was an old operator trick or not, but I found it was the angle of the actual target to the playfield which stopped the ball correctly dropping the target some or most of the time.

To fix it on the machine I was working on, I simply bent the front brackets of the actual drop target assembly back a touch, so all 4 brackets were exactly at 90deg so the drop target was at 90deg to the playfield (if this makes sense)...

See attached pic - red line is your targets now, green is the angle they should be at?  Basically the bottom of the target is touching the playfield, whereby there should be a small gap.