What parameters are you using in your smart playlist (you could take a screenshot of the smart playlist editor to show this)? What happens if you copy the artist (ctl+c in song editor) and paste it into the smart playlist editor? What happens if you use "contains" operator instead of "is" in the smart playlist editor?
What parameters are you using in your smart playlist (you could take a screenshot of the smart playlist editor to show this)? What happens if you copy the artist (ctl+c in song editor) and paste it into the smart playlist editor? What happens if you use "contains" operator instead of "is" in the smart playlist editor?