The chances of getting an armor fossil back is random. Here's a hint: save the game in front of the man who gives you fossils and turn the ds(i) off. When you turn it back on and start playing pokemon black or white talk to the man who gives you fossils(where you saved it) and he'll give you a fossil. If he doesn't give you the fossil you want just turn the ds(i) off again and repeat. (HOPE THIS HELPS) :)