First thing is to insure the fuses to both coils are good. If one is blown the other coil has to do all the work and that is too much for it.

Next is to check the stroke on both plungers to make sure they are adjusted adequately such that they bring the targets up high enough to get all the targets onto the shelf of the drop target frame. If not able to pull the targets high enough adjust the coil position(s) just enough to make up the needed stroke.