I just thought of something else. I don't know if you already know this but it took me a long time to notice. At the top of your PM box it tells you if there are PMs you've sent that haven't been received. I was thinking that maybe the person who hasn't responded to your PM just hasn't been on to get it. I hope that's the case.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226