The part comes out with 2 10mm bolts but, check the arm under the plastic vents and ensure the plastic ball joint is not broken off the metal bar. I had the same issue and it was the ball joint popped out and then the arm would just move all over and not turn the wiper.
The shaft is steel with serrations. The arm is aluminum. Both are tapered. If the shaft spins in the arm, the serrations eat some of the aluminum in the arm, and it can stay embedded in the serrations of the shaft. If you try to tighten it, it may work if you can get enough friction, but the serrations won't dig into the bore of the arm itself. Clean the serrations with the tip of a knife so that it will bite into the arm again, and you'll be like new.