This happens for different reasons.

-Make sure that you are on the correct screen. Check the bottom right corner, you will see the logotype of the screen in which you should be to activate the coloring page experience.

-You have painted too much the coloring page. Every template includes some marks that are needed to bring the experience to life. You must keep them uncolored.

-The coloring page is too damaged (crumpled, wet or broken).

One advice: avoid strong and dark colors, they make the recognize of the coloring page harder.