Nexus users running Android 6.0 and above are reporting Google sync issues on their device. The issue causes apps like Gmail, Inbox, etc. from not syncing in the background, causing the owners to miss email notifications.
The issue seems widespread enough, with a thread on Nexus forum and AOSP issue tracker each, and several people reporting seeing the issue on their device. Google employees have been made aware of the issue, and are working on it.
Some have observed that the issue could be due to the Doze feature in Android Marshmallow, which suspends certain tasks in the background while the device is motionless to save battery. Excluding these apps from the Doze list does seemingly solve the issue for some.
It is not only nexus devices who are facing this issue, also users with lollipop and other brands like HTC, having the same issue.
since lollpop getting worse with longger boot due apps optimizing every time phone booting