I also had this problem and so far I figured out two reasons why this happens:
1) You switch the 'phone or the Wi-Fi off before finishing sending a message (it happened to me when traveling on the London Underground network, where the Wi-Fi cuts off between stations).
2) When the memory in your handset reaches a critical stage, with less than 300MB of "Available" space left - usually when the Hard Disk icon appears, which means "Not enough storage space".
The first problem has disappeared since last year (2015), looks like they fixed it somehow.
The second problem is manageable, if you watch out for this icon, delete the files you have sent / saved elsewhere and the message "Cannot Sync" will not show anymore and the Outbox in GMail will suddenly 'flush' out all the waiting messages into the "Sent" folder.
Hope this helps.