My first thought would be to reduce the contrast on the leaves -- the strong dark tones on the leaves diminishes the glow that middle tones can have. Take a look at the leaf directly above the star design on the vase and imagine a similar tone on all the leaves.
But it may be an over-all lighting issue -- too harsh of a light that is in conflict with the inherent softness of the subject.. Perhaps a soft fill light might help. I am mentally picturing an upward explosion of tones -- a slightly darker toned vase, middle tones in the middle, with the whites of the flowers as they are now.
Thanks for posting your images! They have been inspirational. I might disappear into the studio tonight and photograph (8x10) a couple of artichoke flowers that I have had sitting around since the end of the summer.