McCoy struggled with it in Journey to Babel as well (although my memory fails me on whether or not he actually couldn't make the salute or just complained about it).
http://tos.trekcore.com/gallery/albums/2x10/Journey_to_Babel_018.JPGHe tried to do it(I don't know if they showed it, someone will have to check), and complained, "That hurts worse than the uniform."
http://tos.trekcore.com/gallery/thumbnails.php?album=62
Yeah, I looked it up.
It's interesting to note that Kirk simply bowed instead.