The truth is, you will not always get their best energy and that is because you are still teaching them to form the habit. As long as they can admit they could give more, then they should be willing to get after it in the next practice.
Sometimes the best cure for a lack of effort or energy is a break away from the normal routine to try something new. I sometimes like to throw something different at our players to keep them on their toes. One day I told them to come to practice with their favorite sunglasses. When they showed up we had music and literally had a dance off on center court before practice. Our manager voted on the winning team and the girls had a blast! The icing on the cake was them seeing me - their head coach get out and dance too and it made them all laugh. Practice that week was full of energy and effort....our team had fun getting better.