First, if you haven't discovered Evernote, yet, you may want to do so here. It is the handiest and most powerful little productivity tool that will work across all platforms.
But, like some apps, it can have it's issues. Today, the iPhone, which I had used to snap a document photo, would not sync.
So, I did the next best thing to trying to figure it out myself and Googled "evernote iphone won't sync." (Yes, Googling is that easy).
I came across this very useful article. Useful because the author's very first suggestion solved the problem. Turn the Airplane mode switch from off to on and back to off. Yep, that was it. You will have to read the rest of the article to see what else you can try.