Bookmark this website. It's your new best friend.
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Controlled_Lamp_Problems
I'd guess that the problem might be a bad TIP107 transistor on the board, or it may be a bad or mis-wired diode on a lamp somewhere. This website will help you perform the necessary tests to figure out exactly what the problem is.
Also, you might want to check out this thread: https://pinside.com/pinball/forum/topic/tech-ijw-lamp-matrix-problem