Well I can think of a few reasons, first most fish know a plant and most community fish don't eat them, you presenting a leaf is just like the leaf of a plant in the environment or tank. They also may not be herbivores but i think it's mostly the way you fed it.
Try chopping it up first so they can actually eat the small pieces.
I feed my fish crushed peas sometimes which they like. So try that too and you may have more success.