I just figured mine out. When you go to Settings>Accounts & Sync and the sync settings are already check-marked.. click on your gmail address under the "Manage Accounts" bar. It was only syncing my Contacts and Calendar. "Sync Gmail" was unchecked, so I checked it.
I am having this issue currently- both of my gmail accounts (one is a Gapps account in full disclaimer) are not syncing.
Everything is turned on properly and set correctly. It is only sporadic when it happens.
Some days it will be in the afternoon nothing new will come in, other days like day it won't sync all day unless I refresh!