I'd start at the beginning and count the fish numbers in the lake? How many are there? Say you only ever see 10 fish, then double that as an estimate. Twenty fish will not eat loads of bait and possibly they simply have too much food? Just a guess and trying to find some answers for you.
If it is a case of too much food, then you may have to fish with something very tempting and visual, like a single bright bait IN THE RIGHT SPOT. Try stalking and watch the fishes reaction to your baits. Try a full days floater fishing walking round putting some baits on the wind and watch, then watch some more.
If it's not that, try all natural baits like worms, maggots, and such. It's a start.
Most people seem to suggest different baits. I'd start with finding the fish, counting and watching them feed. That will be your starting point. Carp fishing is a marathon not a sprint.
Dicky.