Mikey's right - age and sex do have a lot to do with it. I found that tropicals were always the preferred treat and if I had enough of those in the ponds, the hardies got to keep their leaves. Buds never ever opened without at least a large bite out of the side . . .
Another thing to keep in mind is that if you move a lily or add a lily to the pond it's always going to be the first one they go after. If the lilies are there and part of the environment and unchanging, they stand a better chance of being ignored.
Snappers are much worse - they chop all leaf and flower stems off at the dirt.