The first thing that jumps out at me is your gallery picture. It might be more eye-catching to feature a picture of the orchids in full bloom. A picture of a generic-looking green stem attached to a ball of dirt does nothing to tell potential buyers who are skimming through a bunch of ads what your plants will look like in their full glory.
My second suggestion would be to perhaps experiment with starting a few auctions at a lower price. Check out this site here:
http://www.mpire.com/research/search.page
Registering and using the service is free. Do some searches for the varieties of plants that you sell, and try to emulate what successful sellers of these plants are doing to get bids. I did notice that on many of these auctions, lower starting prices seem to generate multiple bids on many items.
I might also suggest putting a few items out as strictly "Buy It Now". What would be the lowest price that would be acceptable to you to get for the plant? Add a dollar or two to that, and list it as a fixed price. I often find that many sellers are willing to pay a little more for something if they can purchase it straight away, without having to wait for an auction to end.