Kicking from the knees is a very common issue, especially those initially learning 2bk, and will cause a significant deceleration/acceleration with each kick; leg bends back (or upward) from knee, deceleration -- leg kicks down to extended position - acceleration occurs. Much like stepping on the brakes with each kick. 6bk/flutter, legs don't have a much time to bend from knees, 2bk much more time.
To acquire a kick from hips/glutes, minimizing knee bend, you can start using fins - long fins, not shorties. Feel the flow starting from hip and finishing through toes or end of fin. It's very difficult to kick from knees with long fins, actually nearly prevents you from kicking from knees. Experience the flow from hip with fins and remove and try to feel the same motion without.