Legs not able to generate enough propulsion during backstroke swims.

Hello! I'm swimming the 100 back for Sections this week, and I've noticed that some of the better swimmers that beat me in the last race tend to have a lot of white water during their kick. This is mostly because they're able to get their feet near the surface, allowing for a nice kick. However, I've noticed that my feet tend to sick a little too much, and not allow me to do that. I've tried just kicking in streamline position on my back and I do really well with that, keeping up with one of my fastest friends. Combining this with my arms, my feet immediately sink. No, I'm not sitting in a chair, and I keep my head tilted back looking at the sky. I've seen that some people say to 'engage your core' but if so, how might I do that? Should I tilt my head back even more?

Thanks in Advance.