were you in a party, but nowhere near the rest of it, or perhaps the party leader? I had the same issue when my GF and i parted ways, she started in Gridania and i in Ul'Dah, after some leve quest burning we split up to do the story quests and i found the mobs i killed gave no exp, nor did the ones she killed. Perhaps that is the reason.
Kazamoto Fenrir Server LS=Myrmidon
Kazamoto Futatabi ~~~~ Rabanastre Server ~~~~ Myrmidon
GLD15 Other DoW10 BSM15 And more to come!!!