I think the trouble is, is your lighting. It looks very dark and flat. Your focusing doesnt look too sharp either.
I think the third one is the best, as you can see textures.You need to think about composition more though, and get in really tight, as some of them have a lot of wasted space around them. Just show the shells and their reflections and cut everything else out.
Set this up outside or near a very bright window. Make sure your camera is on a tripod and that it cant be knocked or moved if stood on a wooden floor when you walk around. Get in nice and close and try again.
The shells have some lovely textures, and you need to bring them out, by using side lighting. Or try using reflectors (lust a piece of white paper will do).
As an experiment, you could set it all up at night with the lights on. When your happy with the composition, turn the lights out and open your shutter on a long exposure, using trial and error (2 secs to 30 secs), then using a small torch, paint the light over one side of the shells.
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