How does mission XP work in Borderlands during online multiplayer? My friends and I have noticed that sometimes only some or one of us will get XP / Credit for a mission.
You can only get experience for a mission once per playthrough. If you completed the mission by yourself and then replayed it in co-op, you would not get the experience a second time. If you complete a mission in co-op, it will count towards your single player game for all players involved, if they have not already completed it.
It is also possible that you are not eligible for the mission, which means you have not completed the prior main story-line missions and are not far enough ahead in the game to get that mission by yourself. You will not get the rewards when another player turns it in. If this is the case, it will say so next to the missions and will not track progress for the mission.
One thing to keep in mind is that Borderlands is VERY particular about people being on the same missions. If you are a mission behind everyone else, you can do all the missions they do and get credit for none of them.
If you are noticing that some people aren't getting XP, absolutely make sure everyone is on the same missions. We used to either have characters we only played together or we'd make sure to match up in single player before hopping in.
I'd wager dollars to donuts that's what is going on. If you really are all on the same mission, everyone will get XP/credit.