If you’ve been out in the field all day without a solid internet connection, you can continue to sync when you get back online. The total number of items that need to be synced will appear in a small blue circle on the cloud icon.
In order to upload to the cloud and sync, make sure you have a strong Wi-Fi connection. Once you do, your data should begin syncing automatically, if not-- you can tap on items individually to jumpstart the process.
Check both data and photo streams to make sure they’re up-to-date.
If you have a lot of photos, they may take several minutes to upload.