Check these. Open Gmail and click the menu button in the top left, then scroll to the bottom and hit settings. First go to general settings and find manage notifications. Make sure they are turned on. Then go back to Gmail settings and click on your email account. Scroll down to notifications section and click notifications, make sure it's set to all. Then click on inbox notifications and under sync messages choose either all or 30 days, you can also check the box next to notify for every message if you want. Then close out of Gmail and open up your phone settings and go down to accounts and first make sure that automatic sync is on. Then find your Gmail account, click it, and then hit account sync and make sure all or at least Gmail is enabled. If that doesn't work then you can go to settings->system->reset options and then reset app preferences.