My armrest did precisely the same thing. I took it off the seat and took it apart. I took digital pics to document the whole exercise and do a write up, but when I got the pics from the small screen on the back of the camera to an actual computer monitor I found they were almost all a little on the fuzzy side. Not useless, but not great.
Anyway, the deal is that there is are two roll pins pressed into the bar the armrest mounts to the seat with and one of them has worn through and broken. You have to remove the leather cover from the armrest and pull the mechanism out (I slit the leather on the inside of the armrest under the plastic cover). The mechanism is swaged and rivited together and the rivets and swage points have to be drilled out. Then you can pull the cast pieces off and open the sheet metal enclosure to access the broken roll pin. Then it's a simple matter of replacing the roll pin and putting it all back together.
It's not for the faint of heart, but it can be done in a couple of hours. I managed to do it and after putting it all back together it looks as good as it did before. I can supply you the pictures I took if you like. That will fill in a lot of gaps in my explanation.
BTW, the left and right side armrests are mirror images of each other and aren't interchangeable.