I knew some guys in a mildly successful rock band. They had a few hits, but got really really tired of being asked to play them all the time, year after years.
So I kind of think that there's some of that going on. Several years ago, I read the first few installments of Sara Paretsky's V.I. Warshawski series. But then I pretty much lost interest all at once; Warshawski no longer seemed interesting and a lot of the same sort of stock situations came up in all of the books.
Paretsky could write when she wanted to, so my theory was just that she had no more to say about the character, but felt obligated (either contractually or monetarily) to keep pumping them out. (I haven't read her in a while, and according to wiki she took a 5 year break in the 90's, so maybe she got her mojo back).