why does an icloud backup password message pop up on my ipad?

In the past week every time I use the slider to unlock my iPad in the morning I get a message requesting my iCloud password for backup. I do so, but in fact when I checked there has been no backup since 3/30. What gives, and how can I prevent this popp message. I am willing to grant my password as a given for backup purposes but cannot find out where to do so.