Tie Up Loose Ends
A writer's job is to pose questions so the reader reads on to discover the answers. Sometimes writers pose so many questions that when they get to the end of the book they accidentally forget to answer all of them.
Books that are written as series will often pose some questions to be answered in an upcoming book. However, even in a series, each book has a set of plot problems and issues that must be tied up within the course of that novel.
Look at all the internal and external problems you gave your main and secondary characters. Have you solved all their issues? Were there two people who had a misunderstanding during the course of the story? Did you resolve that issue? This doesn't mean everything needs to be tied up with a perfect bow, but the characters need to come to some sort of a conclusion. Have you had a character ponder a question or even search for a lost item? Amazingly, when readers close your book, they will expect your character's curiosity to have been sated, and either that lost item to have been found, or the character to have accepted it as lost forever.