Is one of your party members at too high a level for the enemies you're fighting? It's just like FFXI was before level sync was added - exp gained is determined by the party member with the highest level. If you have a level 50 player in your party and you're fighting level 5 mobs, then you're not going to gain any exp simply because the level 50 player is there (if I remember rightly the range for the most exp is around five -six levels between lowest and highest level in the party).

Also old style exp parties are not viable in FFXIV - you can still do it of course, but it's deliberately been made not to be as efficient as it was in FFXI. Instead you're meant to do FATEs, side quests, levequests and the Hunting Log for exp - 'content based' exp as Yoshi-P has termed it.