This question is slightly less straightforward than the title implies, and requires a little explanation.
Firstly, I am an entirely self-taught writer. Because of this, my methods are probably a bit different than 'the norm' (if there even is such a standard). For example, I believe that the message of a novel is its most important aspect. I strive to form a novel from a message alone, so that the story naturally conveys what I am trying to say to the reader. Everything else - characters, stakes, and especially plot - flow from the message. This may or may not be ideal; it's simply what I've found works the best for my writing.
Because I write my novels this way, the message is a very important part of my development process. It's where everything begins, and it's something I have to develop correctly the first time, or everything that springs from it will ultimately not work to its full potential.
Question: Can I repeat the same message over a series of novels? Because the novel springs from the message, my concern is that the novels will all follow the same plot, and so become boring. If I can use the same theme, how can I ensure that the novels are different?
My reason for confusion is that Harry Potter seems to have done this quite successfully. The message of Harry Potter, as I see it, is the power of love, and how those with love will always triumph over those without. This message remains the same throughout all seven books, and at least the first six books seem to follow the same general pattern. (Dursleys, school, steps to solving main conflict, confrontation with Voldemort, resolution.) This clearly did not detract from the books, hence my confusion.
Note: Please be clear: I realize using a message this way is vastly different than the majority of authors. That's not what my question is about. My question is about using the same theme over a series, when the plot springs from the theme directly.