Tailgate sometimes come open on its on even when it is shut hard. Any suggestion on why it doesn't lock properly.

 

Replace the latch.

 

Mine did that once but hasnt done it again. i would suggest removing the latch mech at each end and cleaning them.. check for rust on the arm that clicks in to hold latch closed.? im betting its stiff and isnt springing back in all the way to hold the latches shut

 

Cleaning and lubing the handle wouldn't hurt either. That's what was binding up on my truck. (though mine is second gen.)