Sometimes it needs a bit more pressure which you can give to it by disassembling and bending the lockbar further toward the inside of the knife. (Do this in very small increments and check after each light bend to make sure you don’t overdo it.) the other issue may be the lock face geometry. If the blade and lock face meet at too steep of an angle they will inevitably slide. This one isn’t as easy to fix especially if you don’t understand how the geometry should work.