I can't back up my Iphone 6 in icloud

My last back up in icloud was made back in 2014 with a 4 model because I had too much things on it. Now I just bought the 6 plus, cleaned up my files, but when I try to back up it still does not work.

I still have 4 GB free in icloud and have only 2 GB to be backed up. What can I do about it? Thanks!