I have had good success replacing the picker cup when there are plunger issues. I prefer to just throw parts at a machine till it works but there are those out there that like to diagnose the exact cause. If that you I suggest downloading the repair manual and follow the trouble shooting flow chart in the back of the manual. The flow chart always ends with replace the main control board but along the way there is much to learn. Good luck.
PS
D&S Is a good source for info
https://www.dsvendinginc.com/t-dixie-manuals.aspx