What exactly are Meg and her friends fighting against? That becomes clearer towards the ending. As it stops and starts on the way to a conclusion, the script, which was written by Jennifer Lee (Frozen) and Jeff Stockwell (Bridge to Terabithia), relies on fantasy mumbo-jumbo to paper over fairly rote screenwriting conventions. Where the filmmaking finds joy and exuberance in the wonky premises of the material, the writing only finds problems to solve. Weighed down with the obligation to replicate familiar hero's journey tropes, the tension stalls. It sucks the zany spark out of everything.
For example, when the the Mrs.'s inevitably fall away from the story, giving our young heroes the chance to travel, flirt, and get into trouble without adult supervision, the story beat is delivered with a contrivance. After Meg's desire to find her father prevents them from "tessering" back to Earth for reinforcements, Winfrey and her sidekicks claim that all the "tessering" has drained them of their energy and now they must head back without the kids. Instead of feeling like a surprising plot twist or a natural turn in the story, it scans as a set-up.