So I found this nugget on the PSNP forums:
"The theories about this are true: Only people who bought the game in the first months of Vita (Feb- Jun-ish) can receive incoming challeges. The rest must rely on sending a challege to one of these "early players", and hope that they overcome it and send you back a reply.
If you get that reply (it took me more than a year...

), then it's just a matter of playing the challege again, get a better score than your friend, and the trophy is yours."
So we need Bums to play our challenge and reply with one? Bums? I'll send you a Stinkor car air freshener if you can help a brother out.