I have the Droid Incredible and I've been having the same issue, it's really annoying. Earlier last week, I was receiving the "Low on Space" error and I finally got that fixed hoping that it would fix my gmail as well since it all happened at the same time....well, it didn't. After I fixed the "Low on Space", at first my gmail wouldn't even go to my Inbox and went to the All Mail label so I wasn't getting any notifications, etc. Now, that I have it fixed and going to my Inbox...it won't refresh unless I do it manually.
I hope someone knows a quick fix to this.