Q: New iphone not automatically backing up...back up on, plugged in, wifi connected and screen locked...

Hi, I just got an iphone 6. I restored my back up from my old iphone 5 on Sunday. Auto back up is selected, and I pay for extra storage to share with my daughter. I just got a little pop up saying it hasn't been backed up yet. I just assumed it was backing up like my old phone did. Every day it's connected to wi-fi, plugged in and locked at least once for several hours during the day. I don't know why it's not automatically backing up. At night, I have a white noise app I use but I'm pretty sure my iphone 5 still backed up during that. Even so, there have been 2 nights since I got it I didn't use the white noise, and it's was locked all night and still didn't back up. I just had to manually back it up now. Why would it not be automatically backing up?