DIAGNOSIS The fence locking mechanism likely needs adjustment. There's an eccentric cam (an off-center pin) under the front fence lock that controls how tightly the fence grips the rail when you clamp it down.
FIX You'll need a hex key. 1) Unlock the fence and slide it off the saw. 2) Flip it over. You'll see the front locking lever. Near its pivot point, there should be a small set screw or hex-head eccentric pin. 3) Turning this pin slightly changes the clamp's reach. Make a small adjustment (1/8 turn), reinstall the fence, lock it down at the front of the rail, and check for square at the back. The goal is for the fence to lock tightly with moderate lever pressure without you having to lift or push it to align it.
IF THAT DIDN'T WORK Check the rear of the fence. There's often a setscrew or adjustment for the rear guide block that rides against the inside of the rear rail. Loosen it, square the fence to the blade, then retighten it so the block holds that alignment.
HEADS UP Always lock the front of the fence first, then the rear. And always check for square with a reliable square after locking—don't just trust the scale. A fence that moves during a cut is a major kickback risk.