Try right clicking the play list and then choosing "Edit Smart Playlist" and make sure it set to "Songs in the last 41 days" (or however many days you want) and "Live Updating" is checked.
If "Edit Smart Playlist" does not show it may be because it is not a smart playlist, this can happen if you import your library from other computers. To solve this go to File > New Smart Playlist and then put the settings I mentioned above.