It is DL policy to only serve one cocktail at a time. I guess it was getting out of hand and a few recent incidents have caused the Co. to start enforcing the policy. DL also wants consistency.

So, when you run into a FA who only sells one drink at a time, they are actually doing their job by following policy. Don't take it out on them. Consider yourself lucky if you're able to get more than one drink. That FA is breaking the rules and open to discipline.