Does it award replays via special or match?
If not, then it is possible that the maximum replay switch is open when it should be closed.
http://www.pinrepair.com/em/index3.htm#free
If it will award replays for special and match but not for high score, then it could be the wipers on the reels, or it could be the common wire on the PCB on the reels. Check the wipers and the common wire on the 1000s and 10000s reels if this is the case.