This is a good question. I think it’s really easy and, in some cases, quite common for writers to, as you put it, stagnate for a number of reasons.
First, most writers will gravitate toward a certain voice, a certain tone and mood that feels natural to them. I know that with my style of writing and editing, I will end up with punchy, fast-paced text that doesn’t include elaborate descriptions. It’s just the way that I’ve developed my voice as a writer. I like to explore different ways of writing—first, second or third person, present and past tense, different genres—but I do have a distinctive voice that will probably come through in all of my stories.
Similarly, the themes of a story will typically reflect what big ideas or questions the author is exploring in his or her life at the moment. I think that will almost always come through in some way in the story. Personally, I read a lot of philosophy which makes me ask different questions about the meaning of life as I work on each book so that there isn’t too much repetition of moral dilemmas and thematic material.
When I write my books I try to have a different flavor and plot for each one. While all of my novels are thrillers, each has a different feel:
The Pawn: psychological suspense
The Rook: techno-thriller
The Knight: gritty crime
The Bishop: political intrigue
The Queen: terrorist activity
Opening Moves: intense suspense
Placebo: science-based conspiracy
By purposely setting out to create a slightly different mood and plot focus, it keeps the stories fresh and helps me avoid writing cookie cutter novels that are basically the same in plot structure.