I have this same problem all the time. Everyday. However I still manage to post all my posts in the category I want. How? After doing several tries, (first try , then go BACK), I notice what is the COLOR of my wanted category. Then I simply click on the edge of the circle with the color I want. Sometimes not all circles are visible - then I try again and again, and try to click very very quick, while the circles are still moving, spreading out in both sides. Give it a try. It works! At least - for me