Can I suggest you take off the 'faulty' wiper arm.
Put the stiff bit from the nut to the hinge in a 'vice'. You're looking to straighten this section, to get it to apply more pressure to the windscreen. A curve has crept into this section over the years, and maybe more so during the taking off of the wiper arm when the windscreen was replaced.
It works, BTDT, if you're not sure, get a second-hand wiper arm and do the 'repair' on that.
HTH.
