She needs something dramatic to happen where she ends up trending. Maybe beating Serena in an epic 3-setter to win a major or if she says something controversial.
Also, when it comes to Twitter, it's the celebs that tweet frequently and are creative, colorful, and funny with what they say that generate interest and build followers faster. And being a teen idol would have helped her.
One thing's for sure, it they had Twitter in 2004, Maria would have blown up after winning Wimbledon. Since she isn't the fresh new flavor, maybe people just take her for granted and don't feel the need to follow her.