First, ask for them to pay attention, then ask them what about the game isn't holding their attention.
From your own observation, what parts of the game are they tuning out on? What can you do to minimize those parts of the game or make them more interesting?
The biggest issue at our table is players not paying attention when others are acting out their turns. Our DM is almost at the point of taking cell phones away LOL!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.