I bought some clams off ebay, a few arrived DOA but most were fine. I put them in a container of sand and most of them burrowed, but a few of the larger ones did not but they are still alive. They stay slightly open and close tightly if the are disturbed. To be honest I don't know the species but here is a link of the ebay.

http://www.ebay.com/itm/25-LIVE-FRES...item3ccb062803

Any opinion on why they are not burrowing?

I have a 20 gallon tropical aquarium, full of healthy plants and fish.