I had this exact problem recently. Cleaned the drop targets, installed new ones, put it back in the machine and they wouldn't reset. I had unsoldered the solenoid to do this. What it ended up being is I had my solenoid wired backwards when I put it back in.

I should also add one of the wires came unsoldered from one of the drop target switches. I think I soldered this back on thinking it would fix the issue and it didn't but I may have fixed both issues at the same time and I'm just not remembering correctly.