Is it new?
My topbox latch was a bit flakey for a week or so when I first got it while the seals settled in. Works fine now.
The setup used is a little annoying. You insert the key and rotate 90o to unlock. It should NOT pop open yet. Press red button to pop the latch. You cannot take the key out when the lock is in open position, so you have to lock it at all times when closed. Why?
Gilles, if yours is new, make sure the top part of the latch is fully inserted before pushing the bottom part of the latch in to lock. Maybe it is not fully "hooked in". give it a bit of time and if still doing this, I quess it is misaligned and I would take it back.
I just reinstalled the latch. I'm following the directions. Same thing is happening.
Just to be clear: the top case pops open half way when I turn the key to the open position. I say half way because I need to push the case down then fiddle with the latch to get the clam shell all the way open. The latch that secures it to the luggage rack stays put just fine.