Sounds idyllic... I am no trout expert but it could have been they were going for emergers verses adults. If they seemed to be predominately smaller boils without a lot of fin breaking or snout popping, try a half emerged mosquito-larvae looking fly and dress it so it sits just under the film. I often get the hit while stripping in tiny little twitches.
When all else fails I go to a gnarly bead head wolly bugger in black #10 or even #8 and that somehow seems to weed out the aggressors. Another option is to fish a nymph in the top 3-6 feet with tiny twitches. My son will attest to that, he landed a 16" rainbow last time we fished that way under the rises.