When writing a series, be sure to note at the conclusion of each volume (except the final one, of course) that the story will be continued. Include the title and link, if it’s already written, a potential release date otherwise. Without such information, readers may think you just got tired of writing and quit, leaving them frustrated with regard to what happens if you ended with a cliffhanger. If you didn’t, then readers may not realize that the story will be continued. If you know it’s going to be a series when you finish writing the first book, go ahead and put “Volume I” or “Book 1” (or something along those lines) on the cover, another clue for readers that there’s more to come.