On the main site David Silver mentions that wood and stone are superior materials for making a ball for study, and he also mentions that materials like plastic and rubber are inferior and act as insulators.
Rubber is a natural material, at least when pure, so is it some other aspect of rubber that makes it an insulator and thus block energy when practicing with even a solid natural rubber ball?
Of course this is merely an academic question. I purchased a wooden Tai Chi Ball from YMAA, and I'm very happy with it. I see no reason to replace it! I was merely interested in the influences of different materials.