Lock the blade open and try to move it. Any movement back and forth along the blade's arc, called lock rock, is a fault. Side-to-side movement is usually a loose pivot rather than a lock problem. A liner or frame lock bar that has traveled to the far edge of the tang is worn and needs attention.
Push the spine gently toward the handle and then away from it with the knife locked. Movement in that plane is lock rock and means the lock face or the tang has worn. That is not adjustable at home.
Push the blade sideways instead. Movement there is blade play from a loose pivot, which usually is adjustable. See tightening a loose knife.
On a liner or frame lock, look at where the lock bar sits across the tang. New, it should engage somewhere between a quarter and halfway across, leaving room for wear.
If it has crept to the far edge over time, the lock is near the end of its adjustment life and needs service.
A lock that slips under hand pressure on the spine is not safe to keep using. Stop, and contact the manufacturer; most brands we carry cover defects in materials and workmanship.
A stiff lock that is hard to disengage is usually lock stick rather than a fault, and often settles with use.