My puppy does not seem to have much prey drive. What can I do?

I foster military puppies from Lackland--Belgian Malinois--and we are supposed to encourage prey drive in them. This video was helpful to me because it described positive ways to do this and suggested that the puppies may need time to develop. My present puppy (15 weeks) will chase toys but doesn't want to bring them back to interact with me even when I wave an identical toy at her to get her interest. I can see that it is important to not pressure her and to keep this activity fun.