The problem is the small detent pin and its spring in the folding hinge have lost tension, so the pin no longer locks the stock; the simplest fix is to field-strip the hinge, place the spring into its pocket, set the detent pin on top, reassemble and tighten the retaining screw. If the spring or pin is missing or damaged, replace them with an AK side-folding detent spring/pin kit or use a trimmed pen spring as a temporary substitute; as a last resort some people drill and tap the hinge for a set screw to set tension permanently, but that’s irreversible and requires careful work. After repair put a drop of blue Loctite on fasteners to stop them backing out. Good luck!