If at any stage of the quest you fail to serve drinks fast enough, simply talk to Sam again to redo that part of the quest. You don't have to start over from scratch either because the quest keeps track of how many satisfied customers you had from previous attempts.
A good tactic is to just serve as many drinks as you can to one customer at a time. This seems to make it update faster.
This article refers to events, personae and activities only present in-game during the annual Brew Day celebration, which comes to Norrath each year the week of March 17th.