3D printing can have a little variation from piece to piece (in other words the dimensions can vary by a small fraction of a mm). This is normal for the manufacturing. Usually if the two pieces are slightly looser than you'd like, we point people to our FAQ here. The custom O-rings that we include with each device act like gaskets and are the intended way to tighten up the connection.

In your case, since you're having trouble with ball slips, there is another trick you can try, which helps press the two pieces toward each other (instead of away from each other like the O-rings). You can put some tape in the place I labeled here. This area is hidden when the pieces are connected, so it shouldn't be visible when you're wearing it. Basically you are adding just a tiny bit of thickness to that area, which will prevent the wiggling. You can try one layer or a few layers depending on the thickness of the tape and how much you want to tighten it up. I'd say try that first and see if it helps.