PokerStars have recently updated the lobby to include some new views. For the table scanner to work the lobby view must be set to default otherwise the scanner will not work.
For the main lobby view the table view option must be selected and for the players window the list view, and not the graphical table preview, must be selected.
If you continue to have problems with scanning on PokerStars please refer to this FAQ for further steps to check.
My Table Scanner Is Not Working On PokerStars