iPhone 6 will not auto sync

I have two iPhones, a iPhone 6 & a 6s. Both iPhones are synced to our MBP. For some time now the 6s syncs when connected but the 6 does not. The "sync when connected" box is checked for both iPhones. If I click "back up now" the iPhone 6 syncs & does a back up.

Note: Last year when I was using the iPhone 6 ( yes same device) & my wife had a iPhone 5s we had the same issue.

Second note: Both iPhones are kept up to date with the latest version of iOS.

Third note: We share the same Apple ID and sync to the same account.

Any ideas? Is there a setting on the iPhone that could effect this?