My green iguana needs help?

i have a green iguana. i've had her for a year, i believe she is about 4 or 5. the problem is she is only about 3 1/2 feet i feed her greens, iguana pellets, and the reptomin soft gel and yet no growth. she sheds at least once a month and is in a large 55 gallon tank somebody please help me i love my iguana i just wanna do things right for her.