Very normal. Has the male gone into the shell with the female? If he has, then that's a good sign that you've got a pair forming. She's just skittish right now about the new environment and will come out eventually to eat. When I watched my pairs, the male would nip at the female to let her know it was ok to come out. She's always the first one into the shell with the male going in behind her. I wouldn't be surprised if the other two pair up eventually. I found them very easy to pair, and it's good that you got two pair. Very entertaining fish if you can take the time to sit and watch them. Are there other fish in the tank? I found them helpful in bringing them out more often.