I have never heard of doing that, how cool!
If the shells are powdery, they might be too dry. When you say "slsa", does that mean sodium lauryl/laureth sulphate? If you are adding SLS to the shells only, then the shell mixture will have a greater percentage of dry ingredients compared to the insert mixture. This might be making the shells dryer and more powdery. You could try adding more moisture to the shells by adding more oils, fragrance, or witch hazel to your shell mixture. If the mixture is wetter, it will probably pack around the inserts much better.
Or, you could try packing on the shells within a few minutes of making the inserts, instead of waiting overnight.
That's what I would try, anyway.