Biggest problem is that the lighting is coming in from the sides only.
Also, the light isn't being diffused enough. You need a larger "box" and bigger light sources...as well as lighting coming in to light up the front of the piece.
The Marmite jar is the perfect example of not enough lighting on the front.
Make a frame at least 30 - 36" on a side and cover the sides and top with a thin white fabric or scrim or some sort of other diffusing material.
Those stupid little lights shown in that site are almost useless for good lighting.
Remember that the output of those lights are diminished as it goes thru a diffuser, thus the need for more wattage.
It is easier to stop down the aperture of the camera than try to open it for more light at the expense of depth of field. In that point.. for the best representation of your work, you need as much depth of field as possible, which makes more of the piece in focus.
The smaller the box, the more the background is noticeable in the photo... meaning that area where the backdrop goes from flat to up the back, which usually seems to split the piece in half visually.
The idea is to either make the piece the only thing you notice or make it so the piece is on something that ADDS to its appeal, like a log or a brick wall or some other thing.
I remember a while ago on eBay, there was a guy who sold minerals and crystals and he had his girlfriend hold them so all you saw was the rock in her hands and her boobs above it.
Definitely added to its appeal!!