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There are many reasons for this. It could be an issue with sensor, power button or a glitch in the software. If you have any case/cover on the phone, then remove it. Then wait 2 minutes and turn it on. Test without case/cover.
Also go to Settings > Display and turn off Smart Stay. Then Set screen time out to 1 min or longer. Turn on "Keep Screen Turned off", so it won't turn on automatically unless there is a real need (like notification, calls etc)
Let me know the results please. The above steps are to rule out software glitch.
Sorry for the delay Amber. Open Play Store app, download an app named Sensor and run it to test all results. Post the results here please
We can reset all settings and see if it works or not. First download all your important files to your computer or download Dropbox app from Play Store and backup your files to it.
Then reset the settings of your phone. This will reset only some settings and will not delete any files.
1) Tap Apps
2) Tap Settings
3) Tap Backup and reset and tap Reset Settings. Then reset it.
Does it work now ?