This happened to me once, too. What happened is that my veteran account leveled, which made the quest I was turning in trivial to him, and therefore, the 300% is disabled for you other toons. Either way you turn-in, you're going to have the same issue, I think.

After this happened, I always make sure I turned in the lowest level quests first when turning in a batch so I didn't grey anything out, if possible.