iPhone 6s cannot backup to iCloud, please help!

- Been using iPhone since 3gs, buy a new iPhone each year, and restore from the old one to the new one

- My iPhone 6s is the latest update version iOS9.2, and all the apps are up to date

- The last backup from iCloud was 31/5/2015 7:05am (it was on the iPhone 6 by then, I think it cannot back up since I updated)

- Even to backup to my Mac, it always take at least a few times to be succeed, always say it was disconnected

- From the Backup in iCloud of my iPhone 6s, it shows "Last Backup: Never", but from iTunes it says 31/5/2015

- It also says "The last backup could not be completed", every time I pressed "Back up now" after few minutes it just stopped automatically (before updating to iOS 9.2, it took 1-2 hours or few hours to back up, but at the end it also just stopped with no sign)

what can I do please?