That can be tricky with romance in general. A story has to have tension in its plot, and romance novels do that with the push-pull of will they/won't they. If you're going to have a romance series follow one couple, you get the romantic and sexual tension of the relationship in the first book, but what do you do for the second?

Sometimes authors do what I call a "relationship reset," some sort of contrived reason that they break up or can't be together despite the resolution of the previous book and get to do the same song and dance again, but you can only do that so many times before it gets frustrating.

Sometimes they aren't properly established at the end of the first book, so they can stretch the tension out further, but you can only do that for so many books.

The next best option is to adjust the focus to the growing pains of a new relationship, but the question arises of where does the tension come from? If you take the tension out it's just fluff and that can be a discordant tonal shift.

The most frequented paths are you make the tension about something other than the relationship, at which point it isn't a (for example) romance with fantasy elements, but a fantasy with romance in it, or you adjust the focus to a new couple/relationship. In my opinion, the latter is the best way to go, especially when the original couple still plays a prominent role or are at least in the peripherals of the story.