Mail Notifications Not Showing Up (GMail)

Hi all,

Since I've had my iPhone, I've been using Yahoo Mail on it. Everything was synced. No problems. I got my notifications. All was well. This week I switched to GMail. I synced GMail to my iPhone no problem. I changed it to my default email and turned on notifications for it. However, I am not getting any banners or notifications when I receive mail. I have to go to my inbox and manually refresh in order to receive my mail. Since this is obviously quite inconvenient, I'd like to fix the problem as soon as possible. Any ideas?

Much appreciated!