Platy buoyancy problem?

I have one platy that has trouble keeping afloat. She is the only one in the tank with this problem (there are approximately 20 adult platy about 7 adolescents and approximately 15 newborn to infant fry as well as other fish). She will stay on the bottom of the tank until I put my hand up to the side, then she will swim to the top briefly, but it is a struggle for her to maintain her position in the center or top of the tank. Her tail and fins are working constantly and she soon tires and retreats to her favorite spot on the bottom of the tank. She is about 6 months old and was born in the tank. I noticed this problem from the beginning, but I figured that she just liked it on the bottom. Now, I'm curious as to why it's so difficult for her to float. She seems otherwise healthy, eats the same amount of the same food as the others, and as I said, she has been in the same water since she was born. Does anyone know what the problem is and what (if anything) I should do to make her more comfortable?