I'm having the same issue with Samsung S5. There should be a way to resolve this. My pictures, etc, back up fine, it's just the contacts.
Yes, I just did and all backup for contacts ended on June 11, 2015. That's the last time I tried to change info on someone. The change is there but anything I entered on my phone since then does not appear. My photos, etc are fine, it's only contacts that are an issue. I do get a message hourly on my phone that the contacts backup failed. So, whether I try it manually or automatically, it fails.
Bummer. Mine says failed afterwards, but always ends up working. Verizon tech couldn't explain it.
I'm sure you did, but just to cover bases, when you add a new contact they go under "Phone" and not an e-mail or SIM card correct? I do wonder if the back up just says failed if there aren't any new contacts to add. Just a theory.