Open the playfield. Make sure you can reach both sides of the playfield. Now you can easily check the non-working center drop targets against the working side drop targets. You say they don't go down - you don't give great detail so you may want to provide more detail. All 3 of them? 1 of them?
Also - check the springs while you're underneath it.
My center drop targets aren't working and I've narrowed the problem to a sticking solenoid. In my case, I theorize that the solenoid got hot at some point and caused some microscopic warping on the solenoid cavity or pin (armature). I cleaned it really good with mineral spirits, which helped a little but not enough. My company manufactures solenoids so in a couple of weeks when I return to the home office, I'll be talking to an engineer or two about it. My current solution is to ask the engineers if I can machine off 0.005 from the pin to see if it goes in and out any better.
Our problems may be different, but please let us know how your observations go.