Drooping Showerhead

I have a very large Waterpik showerhead, and lately I can't seem to tighten the wingnut on the adjustable arm tight enough to keep it in position. I see a small rubber washer seated between the wingnut and the arm assembly. Might replacing this rubber washer get the showerhead to stay in position? If so, where might I find replacements?
Thanks in advance.
-F
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