They Didn't Stick The Landing - Disappointing Finales

We love series. Movies, books, TV shows, podcasts - we invest years of our lives, waiting for the Grand Finale where All Will Be Revealed.  Only, when the end comes, it isn't. The ending feels like it should belong to another story entirely or has more dangling threads than a yarn factory. How do we deal with it as fans? How can we as creators make sure we craft a satisfying ending? Warning: spoilers will abound.