Change the sync preferences on your individual folders from 4 days to all and back then back to what you want. Mine did the same thing. Found that to work for me!
-Droid Does
I would go into your settings/applications/manage applications/all applications/gmail. Clear out the cache and the data. That should get it working.
I would go into your settings/applications/manage applications/all applications/gmail. Clear out the cache and the data. That should get it working.