You often have to mess with the latches a little. There is adjustment of the pins, and I found that making them a skosh longer helped make it more secure. Try turning them out about a quarter turn first. Remember you have to hold the nut while turning the pointy part with a big screwdriver.
But the more important "adjustment" is usually on the receiver side. They're sort of snapped together with metal tabs and the springs that hold the the keepers might be a little weak.
I replaced the springs on mine to make them stronger (Danny P showed the way to add a second spring if needed, but I didn't need). With the stronger springs installed, I welded all the tabs to keep them from shifting. Then I massaged the receiver slides with a Unibit, opening them up so they'd let go consistently when I pulled the release. I also took care to de-burr all the parts.
Finally I made sure my clam gasket was taking up all the space between the bottom of the clam edge and the body. I think it's really key to keep it from rattling and allow the lid to have a very small amount of tension on the latches while underway. On mine that means there's a D rubber under the traditional leather bit.
With all that done, the panel gaps are just right and the latches hold regardless of bumps & etc But when the car and the ambient air are hot, it's a little tricky to release. A little downward pressure in the middle of the clam usually does it but sometimes I need to pull the passenger edge out just subtly as well.
I also have home made quarter-turn latches which also work but I don't use them, since the stock latches latch and the locks on the ones I made are a little fiddly.