Smart playlist problem on Ipod photo 40GB

Recently I have noticed that my smart playlists on my Ipod are not updating when songs are played.

Eg. tracks not played within last week that are 5*

Even though the tracks are getting played, they still remain in the smart list. It is only when I sync to my PC and Itunes that the songs disappear.

I have never had this problem on my old trusty Ipod, so this is really frustrating !!

Thanks in advance.

Dave