First step is to disassemble and clean the drop target assembly. Over time, coil dust and debris will gunk up the mechanism and stop it from dropping properly. Lift the playfield and inspect the drop target for any physical obstructions or damage. If none, disconnect the drop target assembly and unscrew it from the playfield. Clean and replace. Easy!

I'd like to add to this a bit...
I was having a similar problem recently, and it turned out the problem was that when the drop targets slide downward, they are guided by a washer that is pressed into a guide/track on the backside of the target. One of these washers was very slightly misshapen, so it did not slide cleanly. Often it would slide halfway down, and then stop.

I fixed it by grinding down the edge of the washer a bit to make it smooth, but if you had extra washers that fit this groove, you could just easily replace it. I also had issues if the washer was too small - it should fit precisely and slide very smoothly. The correct washer size is listed in the manual. And watch out for cheap bulk washers that are slightly misshapen (i.e. too thick, not perfectly round).