I was reading in the wow forums earlier and found a blue posting that confirmed that they are having some weird random problems with certain chain quests not giving triple xp to all party members. According to the post there is not a definitive problem as it is random in nature and sometimes works as it should.

I had this happen twice on my dual-boxers so far. 1st time was on the absent minded professor quest (the escort one where u lead the dwarf around the ruins looking for the artifact). The 2nd was in darkshire on the night watch chain quest. The next to last chain didnt grant triple xp to both only to one of the toons.