Diagnosis
The stainless steel latch pins and the receiver holes are likely dry or have accumulated fine dust/debris, increasing friction.
How to Fix It
Fold the scooter to the unlocked, open position. Apply one drop of light machine oil (like 3-in-1 oil) to each of the two silver latch pins on the stem. Also apply a drop to the hinge joint. Work the mechanism open and closed 10-15 times to distribute the lubricant. Wipe away any excess.
If That Didn't Work
Inspect the black plastic locking cam inside the latch mechanism for cracks or deformation. Operate the latch lever while watching the cam rotate. If it does not move smoothly or appears damaged, the latch assembly needs replacement.
Heads Up
Never force the latch closed. The correct sequence is: fold stem down, press the silver button on the latch, then push the latch lever forward until it clicks. Forcing it can shear the plastic cam.