RE: light biters.....
From what I have heard from pro fishermen is there are a few reasons for fish biting lightly. Once you find that reason you can correct the problem of not being able to set the hook.
One of the things you said was the trout are pretty smart. They could be used to people fishing and are only nibbling at the bait but they feel you on the other end of the line (you also said you didn't have slack in your line). Give some more slack and wait a little longer before trying to set the hook.
Another thing to try is a different bait...smaller hook...larger hook...sharpenin your hooks...longer leader between the bait and the weight, go as long as 5 feet...thinner diameter line, if you have 10 pound line go to 6. Remember #6 test line will catch a 6 pound fish. You can catch a heavier fish on it also, just remember to adjust your drag and have fun! let them take line then rell them in some, let 'em take some more and reel 'em in some...
Have fun!