Diagnosis
The locking latch pin and the hinge joints are dry. Dirt and lack of lubrication cause increased friction and wear.
How to Fix It
Apply a small amount of white lithium grease or silicone-based lubricant to the main hinge pin and the sliding lock mechanism. Work the latch open and closed several times to distribute the grease.
If That Didn't Work
The spring inside the locking mechanism may be worn or misaligned. You may need to replace the entire folding latch assembly.
Heads Up
Never force the latch closed. If it doesn't click securely, the scooter is not safe to ride. The stem could collapse.