OK, Here is the deal with Data East Drop Targets!!!
The mechanism wears on the hinge pin for the brackets that lift the drop targets. This causes the bracket to droop further and further until the coil plunger is so far out of the coil or sort of cam'ed over so that it doesn't move with a smooth motion enough to lift all 5 targets in the allotted amount of time that the coil is on.
The easiest way to detect for this wear is to actually look at the top of the drop targets from the top side of the playfield. Knock the targets down, now check to see if the tops of the targets are flush with the playfield. If they are too low, then the assembly has some wear and you need to make some adjustments to get the targets and lift bracket to sit at the correct height.
Way back at the beginning of this thread AlexG mentioned some washers. To fix the height of the drop targets you need to add some washers to the 2 locations at the red arrows in the following picture. Only as Alex said you need to put the washers under the frame bracket and not directly under the screw. This will move the frame bracket away from the assembly, this in turn will limit how far the lift bracket will let the targets fall down. Sometimes there is enough wear that you will need 2 washers. I don't think I have ever put in 3. Also sometimes the hinge rod holes will be so worn that the assembly is garbage and you will need a new lift bracket. I believe all the normal places are sold out of these original lift brackets.
DE changed the drop target assembly 2 or 3 times after this version. Later ones have a nice locking bracket and adjusting threaded rod for fine adjustments of the height of the drop targets when down.
DE drop target washers.jpg