Thanks for playing! Not a bug (hopefully). Inspiration is meant to plateau when there isn't much decor in the room. Each decoration can give you inspiration 3 times in an hour, each admiration giving slightly less in that window. After an hour the mini can get inspired by the item again. The idea being the more variety you decorate with the more inspired the mini becomes!
Each decoration also has a hidden "taste" value unique to your character, and some pieces are worth up to 4x others, so it's worth experimenting to find what your character loves.