As a previous poster said, your Gmail auto sync setting also controls the auto syncing of any other email accounts you have created in your general email app. If turning on your Gmail auto sync is still not working for gmail or your other email accounts, it is another app's settings you need to check and change. This was the problem I had. I had downloaded and set up my Battery Doctor app two months ago and it's an excellent little app. Normally I could go to the power management app that came with my phone (a Huawei Ascend Mate 2) to switch on auto sync, but all of a sudden the option to turn on or off the settings for sync, vibrate and other power options weren't there. Battery Doctor was overriding it. If you have installed Battery Doctor or a similar battery power management app (or maybe even the app that came with your phone), you must go into its settings and look for its ignore list. It might say something like "add or remove apps". The settings for apps you add will not be changed or turned off when you optimize your battery. Add gmail and any other email apps to the ignored list. It worked for me. If you happen to have Battery Doctor, when you tap the optimize icon it will show you all apps it is about to optimize. Just press and hold the app icons you want it to exempt from optimizing. A pop up will ask if you want to add it to the ignore list...or you can go into Battery Doctor settings to do it as well. Afterwards, double check that your Gmail auto sync is still switched to 'on' and you should be good to go. After that don't ever accidentally or purposely reset your Battery Doctor app to its default settings or the problem will reoccur. That's what I did and it's why my syncing stopped so suddenly.