They do that. For a while mine would only slip if it got very hot. These guys sell replacement plastic gears for $25 each:

http://www.odometergears.com/subpages/cable.html

If it's just slipping, the gears may be OK. Sometimes the the main drive gear for the odometer slips on the shaft. If can be fixed with a drop of glue (very carefully) or by making the shaft rougher and forcing the gear back into place. It makes more sense when you are looking at it apart.