Both of my Android devices have been doing the same for months now. It is driving me nuts. There was another thread about this a while back that said going into my apps-gmail-delete cache would fix it, but it only works for a few hours and then stops working again. I have virtually stopped using my gmail recently as work will no longer allow me to have my email routed through gmail, but the problem still exists.
I would love it if there was an actual solution.