What are we doing wrong?

Our daughter who will be 3 this July listens to me, her dad just fine.  When I tell her to do things, she listens most of the time, and she doesn't argue with me or fight with me about most of the things that go on with her on a day to day basis.  But when it comes to my wife, she won't listen to her, and defies her most of the time and fights with her and then hits my wife.  If my wife tries to put her down for a nap or to bed at night, which both of us usually lay with her and get her to fall asleep and then leave her to sleep on her own.  When I put her down, she goes down usually within a half an hour or less, but with my wife she fights with her and doesn't go down for much longer, if at all and many times, I have to go upstairs and either tell her to listen to her mom or I end up taking over and putting her to bed.  It's not like my wife is doing alot of things different than me, but she just won't listen to her or respect her.  This has been going on for close to 6 months when she was an only child, but now we have a 1 month old as well, and it seems to be the same or worse and is wearing on the both of us.  What are we doing wrong, and how can we fix this before it gets worse.