This anomaly happened when I rebuilt my Paragon years ago - the three drop targets would find it difficult to drop. Ended up being a combination that caused the problem ;

1 - The rubber ring behind the targets was replaced and it was a lot thicker than the original - so the targets would not travel further back enough to drop

2 - The drop target assembly was mounted too far back - I didn't want the art on the new targets to touch the groove in the playfield.

The fix ;

Stretch the rubber ring so that it is thinner behind the targets and thicker elsewhere !
Loosen the screws holding the drop target bank, push the assembly as far forward - away from the rubber ring and tighten.

Sometimes - after rebuilding and polishing a playfield, the ball hits the targets so hard, the target just "Bounces" back and does not drop.

If this is STILL happening, you need to check that the springs (one for each DT) still have tension. I sometimes replace them if I still have problems.