There's always a challenge in continuing-series characters, and writers have struggled with it since the advent of the modern pop-culture novel. Ian Fleming got bored of James Bond after a few books, and you could tell. John D. MacDonald let Travis McGee get old before his time. Robert. B. Parker never let Spenser change much, but he was Spencer, and that was always enough. Lee Child is struggling to keep Jack Reacher from becoming a caricature of himself.
Stevens is still in the early stages; she told me she's not even sure where Monroe is going next. Which is why, I think, The Innocent works, and the next one will work, too — you really sense that the writer is growing with her protagonist in this, the best thriller series in a long time.