Make sure that the cloth strap on the lid isnt getting suck between the lid and the bag. This will cause the bag to seem like its closed but it will pop open.
The latch should stay closed.Are you sure the hooks on the backside of the lid are engaged before closing the latch.
Another thing to check are the screws that bolt the latch to the bag and the lid are not missing and are tight.
When you open the latch nd look behind it (on the outside of the bag) you will see that the latch engages 2 small curved metal retainers at the bottom when the latch is closed, these spread open over time and the latches will pop out of place. Just take a small screwdriver and pry them up a little so the hold the latch in place and you should be all good to go.