It might be egg shaped balls, or it might be that all balls are the same size.
Have you ever ridden up behind your buddy on his bicycle, and let your front tire touch his back tire? What happens?
Same thing happens in a ball circuit if all balls are the same size and adjacent pairs are making contact with both the raceway, and the ball in front. This is why spacer balls are used in some ballscrews. A small ball will not be in firm contact with the raceway, and this permits it to run backwards if it accidentally touches the big ball. Because the small ball is small, it clears the raceway and does not skid and cause a lockup.
You may have seen cutaways of some ball trucks which have a plastic cage within, that is flexible enough to move with the balls and also keeps them from touching one another. You might be able to simulate this with short pieces of plastic tubing between, but that would be a total experiment, and could be frustrating if they catch somewhere
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)