Certified GKH Gardening Expert
If one is doing well and the other isn't, it's probably not due to anything you're doing wrong. If you purchased the plant from a nursery, you might consider taking it back and asking them to replace it.
Here's an article about their best growing conditions: https://www.gardeningknowhow.com/ornamental/flowers/heliotrope/growing-heliotrope-plants.htm
Unless the plant is suffering from an insect invasion, the best suggestion I have is to pinch it back, give it a little food and water and hope for the best.