I'm betting the one that doesn't sell has something wrong with it. Some of the plants are verrry good at hiding or camoflaging bad parts. If you take one of the non-selling ones apart piece by piece (by pruning a little at a time), you'll likely find a dead bit that you've missed. Sometimes you can see a dead part while the plant is developing, but by the time it's full grown the dead part is hidden completely or almost impossible to see. Or sometimes it blends almost completely into the soil or other foliage.

If you really can't find anything wrong, and it still doesn't sell, just trash it--it's not worth tying up a space in your greenhouse forever.

What I did when a plant did not sell I put the price down then it soiled with no problam, then I put the price up again on the same breed, I think the dead foliage was hidden as the others did sell at the higher price.

One of the things that customers look for is unobstructed flowers. Sometimes one flower can be hiding, or partially hidden behind another flower. In those cases, pruning a flower may help get the plant sold!

I wouldn't recommend doing this unless the plant is very healthy or you have upgraded the shears.

I got all my upgrades.