Why didn’t the changeover come?

I'm not actually sure this is explained. The loop's failure to reset is a symbolic loss of innocence for the peculiar children who lived in it. While inside the loop, they were able to hold onto their childhood both physically and emotionally, ignoring the outside world and under the constant protection of Miss Peregrine. The loop's failure means that Miss Peregrine can no longer protect them from the rest of the world, and they must grow up and face its evils, however awful they may be.