I have the same thing going on in my tank. I bought 22 baby IT dats in January and all of them eat pellets except for 1 of them. The ones that eat pellets (massivore and cichlid gold) are about 4" long now, but the one that doesn't is still the same baby size -- about 3/4 inch.
The same thing happened to me when I bought peacock bass. I bought a baby occel from one store and he took pellets from day one and 3 months later, he's a healthy 5-6". I bought another baby pbass from a different store and he never ate pellets. He stayed the same size for about 3-4 months.
So I guess the lesson is: Get your fish onto pellets as soon as you can. It's the easiest way to get them to grow quick.