Messages stuck in outbox of Gmail app, but only for Gmail

I noticed messages were not being sent from the Gmail app. I checked to make sure auto sync is enabled and it is. Any ideas? Beside the message it just says sending. I have other email accounts used by the Gmail app (e.g. yahoo) and they work fine. Also I can send and receive find from Outlook on my computer. I don't know what else information to give so feel free to ask for more. I'm using Xperia Z2 with Android 4.4