As I've said before, I don't mind if these books become repetitive because even when they are in a series, they still work as standalone. So their repetitiveness is forgiven but not at the price of their humor.
From 9th book this series started going downhill in both, humor and mystery departments. But Eleven on Top
was definitely better than previous two books but still not good enough. I'll read the next book to find out if it gets better or else I'm calling it quits - at least for now.