I had this problem too. I bought four mollies, three of which died. Bought another one later on and that died too. One just sat at the bottom not moving, one shimmied near the bottom, and the third starting swimming head up like a corkscrew.
Is your tank cycled yet?
Phone up the fish shop you bought them from and ask what pH, why temp they are kept and and if they kept them in brackish or freshwater.
It could be that it's in shock because of a sudden change in conditions.