The slightly smaller size of the colour bearings is by design. The other bearings hold the weight. The colour bearing, if it held the weight would lose coloration over time. If it keeps falling out, as above, you can punch through that little paper in the hole in the middle of the bottom foam with a small Phillips screwdriver to narrow the gap. Clockwise increases tension. 

WARNING AGAIN: This screw is NOT big. It’s a short little thing. Over-tightening it CAN break your spinner. Minute adjustments will make a big difference in tension.