Endings in stories are what readers are striving to discover. For an author, the end means the long journey juggling a story arc is now complete. But often times an ending is less than satisfying.
Case in point is star adult author John Grisham and his first middle grade novel, THEODORE BOONE, KID LAWYER. A fast moving tale with suspense, a few laughs, and mostly a page turner. But for the 9-12 year-old audience and a few of us older readers the end was missing. There were too many plot points left dangling. Sure there were to be sequels, but I talked with too many middle schoolers who felt betrayed by the loose ends.
Some stories end with a premonition of what might be ahead for the protagonist. Jack Gantos did this with his Newberry winning novel, DEAD END IN NORVELT. Previous plot points were already neatly wrapped up.
Many books for middle readers will end with a reflection by one of the characters that gives a satisfying feeling to what has occurred. Think LIAR AND SPY by Rebecca Stead.
Other ways to end a story might be a famous quote, a lead-in to the next adventure, or a surprise no one saw coming. Try different ones and when your story is being critiqued ask which one seems to work the best. The goal should be to see that your reader will either want to re-read the book again down the road or be anxiously awaiting the sequel. Ahh… easier said than done.