Scubagirl, the acid needs to work at the proteins slowly to soften them up so that when they come together to make curd, it's not tough, but fluffy. Meaning use whey from cheesemaking where the acid slowly build up, or add vinegar in the beginning to a pH of 5.2, and heat up slowly.
Ricotta is actually a rather difficult affair to pull off. If it's not acidic enough, you get hard rubbery curd. If there aren't enough whey proteins, coagulation is poor. If it gets too acidic, it will come out of solution, but your curds will be like grain, and tough. If you heat too high, you get firmer curds and full yield. If not enough, soft curds, but poor yield. It's kind of like mozz in that regard. That's why, IMHO, mozz whey makes for the best ricotta.