Is this an antique clock and is it british clockwork? If so fairly easy to fix, all old clockwork should be alloud to strike at twelve o'clock (i know its a bit late to get this advise) if not you run the risk of bending the (racktail). This is almost certain to be the problem its a thin piece of brass with apin on the end this pin falls on the snail (piece atatched to the hour hand with twelve decreasing in diameater edges hence the name it looks a bit like a snails shell. its the tail of the racktail that is slightley bent.
john
January 2008