It is commonly known that sequels are rarely as good as the original story. I have certainly found this to be true. For instance, I love Disney's Mulan. As for Mulan 2, it's not worth the time it takes to watch it. It was completely cliché and predictable, and extremely disappointing after how much I enjoyed the first movie. I also enjoyed Summer of My German Soldier, but I didn't even get halfway through the sequel.
Some sequels are just as good as the first. I have read Return to Gone-away as many times as I have read Gone-away Lake. The Lord of the Rings is a continuation of the story begun in The Hobbit, and it far exceeds the previous tale. (Series are a different matter, and are of no concern to this topic.)
What makes the difference? I believe it is whether or not the sequel is a story worth telling or if it is merely a story written because the first one was successful and beloved. So often sequels are written simply because fans are clamoring for more, not because there is more story to tell. The result is a flat, cliché story that is nowhere as good as the first one. I know that I am more disappointed with the existence of a poor sequel than the absence of any sequel at all.
I have yet to write a sequel to any of my chapter books. I have had family and friends, and even a young reviewer, beg for a sequel to The Experiment. Even so, I don't see one being written. I ended the story where I did because I didn't have any more story to tell. What is in the book is it. Though my sister constantly asks if Audrey marries Adam when they grow up, I have no desire even to decide that. It is possible I may change my mind about writing a sequel someday, but you can be sure it won't be until I have a story worth telling.
Across the Stars is another matter. I have some ideas for a sequel about Hanna and Sam that I am considering writing as a first NaNoWriMo project this November if I ever plan it. Yet, though I have always foreseen the possibility of writing a sequel about Felix and Sara, it is on hold indefinitely for lack of a plot. I could get an idea for it within the next year, in fifty years, or never, but until I do get a good idea, I won't write it.
I am not against sequels. I rather like them. But my advice is not to write a sequel unless the continuation of the characters' story is just as worth telling as the beginning of it. If you have a good sequel idea, then go ahead and write it. Your fans will be happy.