|MLA Citation:||Bloomfield, Louis A. "Question 1426"|
How Everything Works 12 Dec 2017. 12 Dec 2017 <http://howeverythingworks.org/print1.php?QNum=1426>.
The issue is in how well the ball's surface stores and then releases this energy. The ideal ball experiences only elastic deformation—the molecules within the ball do not reorganize at all, but only change their relative spacings during the dent. If the molecules reorganize—sliding across one another or pulling apart in places—then some of the denting energy will be lost due to internal friction-like effects. Even if the molecules slide back to their original positions, they won't recover all the energy and the ball won't bounce to its original height.
In general, harder rubber bounces more efficiently than softer rubber. That's because the molecules in hard rubber are too constrained to be able to slide much. A superball is very hard and bounces well. But there are also sophisticated thermal effects that occur in some seemingly hard rubbers that cause them to lose their stored energy.