Why its not awake after its goes to sleep mode?

I have new mi max phone from last month. from first day its takes too much time to awake from sleep mode. i have to press power button Manny times to awake the phone from sleep mode and when its awake from sleep mode its screen blinks frequently for 20 to 30 second.. i don't know what to do with it. i updated all the apps and its miui software recently.. still facing problem. i cant takes calls when its in sleep mode.. please help