How frequently are you feeding it? What is the primary food item you feed it? How warm is it being kept?
Growth is dependent on the quantity and quality of the nutrition, the proper temperatures the animal is kept at (for efficient processing of the nutrients), other possible health issues such as parasites, genetics, and even the temperature variants, and at what stage they occurred during incubation. Generally the first two are what would cause the growth variable in juvenile crocodilians.
You don't say what size or species it is, but in general for a young croc, feed it all it wants to eat (preferably whole animals), two to three times a week minimum, keep the water temps in the low to mid 80's F (82-86), provide a basking area that is up to about 100 deg F. at the hot spot, and keep the ambient air temp in the mid to upper 80's F during the day, allowing it to cool to the 70's at night. In addition, be sure to provide a day/night light cycle and hide spots in the tank to help alleviate as much stress as possible.
If you have not read the FAQ at crocodilian.com yet, do so - it is linked at the on this page.
Good luck,
Bill