Labrador is afraid of the pool!?

I have to 5 month old puppies, both Labs, Ellie and Luke. Luke LOVES our pool, he jumps right in the second you say the word. As for Ellie, she'll wade in the first two steps and stay there, no farther. We've been working with them where we coax them to the steps with treats, then once they're standing on the steps we hold them under they're tummy and swim around, we still do that with Ellie, sometimes, she goes on the steps whenever the heck she wants to, but Luke's got the swimming down. Can you please tell me some way I can get her to Luke's level? Is she afraid of the pool? Thanks!!!