Something that happened to me once was this.

Quest was green to all 5 characters.
Acct 1 turns in quest and levels up. Now quest would be gray for acct 1.
Accts 2-5 turn in quest and do not get bonus. Now 2-5 are a full 1/3 lvl behind acct 1.

How to get around this if you know you are close to leveling and out leveling a quest:
Turn the quest in on the refered account first and then remove from the group. The others will now still get the bonus. Only prob is your vet account wont get a bonus but it is better to not get it on 1 account then on 4. You can catch up by doing a gather quest on only the vet account.