Before the KitKat update, I had this same issue with my e-mail account not syncing sometimes. After looking around, it appeared that it was a "known" issue. What I would do, was deselect the account and the select it again and it would almost always sync. Also, using the "sync all" widget always worked...Since KitKat and Sense 5.5 have been on my phone, I haven't had any issues.