Had the same issue when i had my ST pro. Also had the same issue with my AC/DC drops and the drops on my Metallica. Adjusting / adding tension to the spring on the drops worked in all cases.
picture of the "fix" on page 1 of this thread:
http://pinside.com/pinball/forum/topic/unplayable-drop-targets-metallica-pro-anyone-have-a-fix