Potsie not at J&C’s wedding

I find it strange that Potsie was not at Joanie and Chachi’s wedding. I would love to know the reason behind it. Was Anson Williams not available at the time? I know that he was not in many season eleven episodes but I don’t think that his absence was explained. I believe that, had this been real life, Ralph would have been there too but he was written out of the show.

And Howard forgetting Chuck in the toast? I know, I know, Chuck didn’t exist after season two but I still find it funny.