Maybe due to stress, not enough light, poor food quality, or just a genetic defect.
What are you feeding them? (and how old is the food)
Is your light on for enough hours?
Is she being bullied or chased?
Did she come from a reputable breeder?
On a side note, She is the exact same colour as my guppy
Ah, I don't know how good petco is because we don't have any in my country! Would need a second opinion on that.
Try upping the light time to 10/12 hours and see what that does. Is there any way they could see sunlight from their tank position?
Food sounds fine.
If you're seeing aggression, that could well be it! Is she hiding at all or listlessly floating?
She's also looking quite skinny for a girl, does she eat enough?