Update: I had two very large sections with hundreds of image-heavy notes. I moved those to separate notebooks, and my iPhone completed syncing. Now, changes on the iPhone are reflected almost immediately on other clients, and vice versa. So, my suggestion for anyone having issues syncing an iPhone is to move any indordinately large sections to standalone notebooks and then open and sync those separately.
If you’re having issues syncing your OneNote notebooks on an iPhone, you’re not alone. I have a fairly significant notebook, around 1.8GB, that I’ve been having a terrible time syncing on my iPhone 6.
I haven’t found any particular reason for the difficulties. I’d say it was just a matter of time, but it’s been about a week now and the notebook hasn’t completely synced. I know that an update was released in the last few months that enables background syncing (http://www.imore.com/microsoft-onenote-iphone-and-ipad-will-finally-sync-background), but nevertheless it seems to take an inordinate amount of time.
I’ve been working on speeding up the process by accessing each section and section group, one-by-one, until it fully syncs, and then moving on to the others. I’ve mostly completed the process, and each section does indeed sync. And so it might just be a matter of patience. Then again, right towards the end (about 1.6Gb of the total 1.8GB), I’ve been receiving some sync errors that are a little disconcerting.
The worst aspect is that until the entire notebook is synced, new notes created on the iPhone won’t upload. That makes using the iPhone to create new notes more than a little problematic. Oddly enough, syncing the same notebook to the iPad Air 2 works fine–perhaps the 2GB of RAM on the iPad makes a difference.
I’m going to keep researching this issue and reach out to Microsoft for an answer. In the meantime, if you have a large notebook and find that it’s not completely syncing on an iPhone, just keep hitting Sync Now (located in the Share dialog) and keep your fingers crossed.