There are 2 small philips screws that hold the arm onto a metal rotating hinge.
Try loosening each screw about 1/8" of a turn. If these screws are too tight they will make contact with the hinged fixed body and not allow it to move freely.
If this solves the problem then tighten each screw snugly to determine which screw is too long.
Remove that screw and run a file against the end of the screw to remove a few thousandths then try again.
If loosening the screws does not fix the problem then it is a defective hinge assembly.