I think "ignoring it due to being overworked or misaddressed" is the most likely -- and if it's either, try fedex/ups mailing a physical item (like a book or other gift, or food, or whatever -- something which doesn't fit in an envelope, and which is obviously nice enough that 1) dude feels guilt if he doesn't respond and 2) intermediaries will try to pass it along.
I've used this trick quite successfully ($20-50 items from appropriate gift vendors -- Cabelas for outdoor type people, gourmet food vendors for other people).
