Dear Dr XXX,
Sorry to bother you again. Though I sent you an email for about scheduling my visit date one month ago, I have not yet received your reply yet. I apologize for rushing you, but could you inform me of the a date when you are that is convenient? If your future schedule is not decided yet, I would be happy if you could let me know about that.
Sincerely,
Yasuhiro
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It sounds polite enough to me. Good luck.