You've only to look at the case involving the Met when they were all taken in by Carl Beech.
Who couldn't have seen through a scumbag like that ? Answer= the top brass from the Met !!
I fear that may have had rather more to do with how we've evolved, as a society, than failings on behalf of the police. Child abuse is rife. No one doubts it. They've become more so following the horror which was Jimmy Saville. WE have the luxury of being able to refer to people, who 'make' on the back of such, as scumbags but whatever the police may have thought of him (Beech) privately, there were protocols to follow and be SEEN to be followed. The irony is that possibly THEIR gut feeling might have told them such was the case but without something to back it up, gut feelings are worth nothing.