Ha, a couple of years ago, I use to fish a L.A. city park about 100 times a year, maybe more and still couldn't get a line on the place. I think in CPL's, things change more than at a regular lake, i.e. weeds grow, then get pulled out, most CPL's get blue stoned, water changes, people keeping way to many fish in some places, die off's, and the list goes on.
So yeah, that's what makes CPL's such a challenge and IMHO fun to fish..