Seems like your going a little light on your dme for starters to begin with. I usually add 13.5 oz for a 3L starter and ive read that some do up to 15oz for a 3L.
Without proper nutrients, yeast will not "grow" with just corn sugar. You can try adding additional yeast nutrient, but I am not sure how this will react with your expected yeast growth in a starter. I think you would be better off making healthy yeast with a 2L starter or waiting till you have enough yeast to pitch.
First State Brewers
Primary: StrawberryHop Saison
Bottle: Rochfort 10 Clone, Bourbon Oak Pumpkin Ale
Keg: Kolsh, Marzen, Saison, Belgian Strong Ale, Brewskie the Elder,Pomegranate Apfelwien, Rye Dortmunder Export, English Bitter