A lot of variegated plants will produce more green on their leaves if the light supply is too low, Gigi. They need more chlorophyll (green stuff) to produce food for the plant in low light.
I would try gradually increasing the amount of light to see if that brings back the stripes.
That being said, it's also a perennial problem with variegated plants that they really "want" to make more green leaves, and some types tend to revert to all green permanently. Let's hope yours responds to more light.
