Don't push the button. It doesn't have enough leverage to work the stronger spring RA went to to avoid unlocking during dynamic movements until it's more broken in.
Instead, to close the stock (if you're right handed like me...if not, reverse instructions):
Grasp the front of the rifle at the handguard near the magwell with your left hand in a normal underhand grip and turn the rifle slightly onto its "side" at about 45° so that the pistol grip's butt is facing your stomach at that same angle. Place your right index finger on top of the upper receiver just behind the very first top picatinny rail...leave the rest of the fingers on that hand loose. Place your right thumb on the pistol brace, just behind the hinge plate bolted to the lower receiver (aka, just behind the QD socket). Simply squeeze your index finger and thumb together and the hinge will lift upward and unlock....then you can fold it.
To open it from the folded position, hold with the left hand in the same place on the pistol, but this time, keep the pistol more upright rather than rolling it on its side. Place your right thumb in the same place as before, but put all the fingers of your right hand (or just the index finger again if you have an optic or BUIS mounted in the way) on top of the picatinny rail (index in the same spot behind the 1st pic rail, then the rest of the fingers moving forward along the top of the rail)....same thing: Squeeze. It'll lift and unhinge the lock and then you can open the stock up.
If the above method doesn't work, then you may have some sort of burr or something inside the hinge. You can either send it in for warranty or take it apart and look for the problem yourself. It's a very simple mechanism. Your call.