Maybe the reason the costume wasn't added to the slot was because I didn't buy it from the shop...
If that's the only reason, I was careless.
Also, when you change an arranged costume to a different category (e.g., Glossy → SuitLuxe), the existing costume is pushed aside.
Alphabetical order takes priority.
I made the mistake of even changing the costumes I had already purchased into a different category.
I ended up confusing the mod system.