I got this message on my ipad "This iPad hasn't been backed up in 2 weeks. Backups happen when this iPad is plugged in, locked, and connected to Wi-Fi.

If you're still having this problem, reset your device by holding the power and home buttons at the same time until you see the Apple logo, then release.

The system error basically should lead to the culprit of your issue. iCloud will ONLY automatically back-up to the cloud if all of the following are met:

1) iDevice is plugged in to the wall outlet/power source.

2) iDevice is connected to a WiFi network; iCloud does not back up over 3G/DATA networks to limit the data caps on most carries nowadays.

3) iDevice has been locked and innactive - usually takes place when one is sleeping.

Make sure all three of these are in fact in effect. If you are noticing that it has gone 2+ weeks unable to backup automatically, I would recommend navigating to the iCloud settings panel on your iPad.

Tap on Settings -> iCloud

On this screen I would advise for you to scroll all the way to the bottom and selete Delete Account.

*THIS WILL NOT DELETE YOUR BACKUPS, JUST REMOVE THE ACCOUNT ASSOCIATION FROM YOUR iPAD**

After this has gone through, sign back into iCloud and manually do a backup by selecting Storage & Backup and scrolling down to select the Back Up Now option. Once this is complete, use your iPad as you would. Over the course of the next few days, go back to where that Back Up Now option was and below it you will see when the last backup was done. This should be changing almost daily if the mentioned measures earlie in my response have been met.

Should this not solve your problem then due let me know. But to my knowledge, this has solved my iCloud issues with my iPhone 4S and my iPhone 5.