We've had similar issues in the past when we were running a My19 and a My12. What we decided to do in the end was allocate just enough parts needed for the first job (taking into consideration reject rate) so when another job comes around needing that part, the stockroom still had parts to issue. Management didn't want to buy extra parts not because of overstock but just plain stingy unfortunately!
(http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=66541)