Diagnosis
Dirt and lack of lubrication on the main folding hinge pin and latch. This is a high-stress point.
How to Fix It
You need a dry PTFE lubricant spray. Wipe the hinge clean with a dry cloth. Apply the lubricant directly to the silver hinge pin and the black latch hook. Work the mechanism open and closed 10-15 times to distribute the lubricant. Wipe away excess.
If That Didn't Work
The internal spring in the latch mechanism may be failing or misaligned. This requires disassembly of the latch housing with a Torx T20 security bit.
Heads Up
Never use a wet lubricant like WD-40. It attracts grime and will make the problem worse within weeks.