Mine bk2 sheath was very stiff when I got it, a real firm push with thumb was needed - there is a little knack to it you soon figure out. It also has a mighty crack when you put your knife in (especially in quiet woods it seems to echo all over). No doubt some Mall Ninjas /Rambo-wannabies would complain cos its not stealthy. It has slowly slackened off a little, but still a very bit of a tight grip. I don't mind too much as if it slackens/is slackening off over time and still holds its grip - for sure the knife is never going to fall out.
If someone can run by you and steal your knife that easily, they probably deserve it.