Songs won't play on iPod

I recently bought two songs on iTunes. They appear to have transferred to my iPod (they show up in appropriate playlists with album art), but will not play--when they come up in a playlist, the iPod immediately skips to the next song.

I have downloaded songs before and since that play fine.

I updated iTunes, I formatted the iPod and re-synced.

Yet the problem persists.

Help!