No need to apologize. I am sorry my joke was not funny.
No, it's not that. I simply didn't recognize it as a joke. Perhaps an emoticon would help. Before the invention of emoticons, I was routinely misconstrued as being serious when making sarcastic or satirical comments. Sometimes it's only the context of non-verbal communication (tone of voice, etc.) ... or now, an emoticon ... that the difference can be discerned.