Personally, I had at one stage about 10 active quests at the same time, and had only one solvable problem.
It seems as if the characters responses get overwritten by new quests, however, I do not know yet if the overwritten responses get restored when the second quest is finished (I don't think so). For my quest, all I had to do to reactive the overwritten answers was to simply go one step backwards in the quest.
eg Talk to A, he will give you A1, then bring A1 to person B, he will then ask you to talk to C who will give you object C1, give object C1 to B and will then give you the object F and the quest is done.
Now, if you have another quest involving character F, try going back one step and give object C1 to B, and if that doesn't work, go back another step (talk to C), et caetera, et caetera.