My moto g3 phone is facing some problems and issues in the security part

[AC Question]

i updated my phone to lollipop and i don't know suddenly my phone it is not locking itself automatically, though i kept for automatic lock after 15 seconds of inactivity, the screen keeps idle and i dont't know what is wrong with it, i had to manually press the power button to lock it,

i checked the settings everything is in correct manner no smart lock, power button instantly locks, but still i am facing the same issue.

thank you