Q:
The balls don't seem to rack any tighter. Why is that?
A:
The rack's job is to compact the balls into as near a perfect triangle (or diamond) as possible. Wooden and plastic racks have sides that flex under pressure. They can also warp and are not machined into a perfect triangle like the DELTA-13 ELITE. Therefore, the balls are never in perfect alignment with wooden and plastic racks. With many of them, you can't even get all the balls frozen together before removing the rack.
Also, unless you are using Super Pro Aramith balls, you cannot trust that the balls are all the same size. Here's the test: Rack 15 balls with the DELTA-13 ELITE. Place your hands in the back of the rack to tighten the rack. Now, roll the rack along the table with the balls still tight. Are any balls turning? If so, those balls are undersized and will negatively impact your ability to get a tight rack with any rack.
Another possible problem could be the cloth. Over time, the cloth gets worn and the slate underneath gets chipped, especially near the rack from breaking over time. The fabric also gets dented. The balls tend to fall into these hard-to-see dips when the rack is removed.
Lastly, humidity may also be a factor. In humid climates, the balls and cloth get moist and pick up dirt. They tend to cling more and it becomes extremely difficult to get the balls to stay in one place when you remove the rack.