I try to say something with my really poor english, sorry!
Why you are waiting so long to make ricotta?
The whey should be "sweet" not acid, in Italy, normally, we make ricotta strait after the cheese.
If you want to make from mozzarella's whey, normal too acid, you must add some c.soda... I never used it!
I live in the north Italy but my family come from Sicily and there the ricotta is made in a little bit different way.
The whey is strained and warm up to 65°C ;
Ad about 1L of fresh milk every 5L of whey and some salt ( some people ad water too);
Stir , scrape de bottom, slowly until 80°C, no too quick.
At this point some foam rise, scoop away!
If you use whey from goat or sheep's milk at 85°C ricotta rise whiteout acid, normally!
But if use whey from cow's milk you need acid ( lemon juice, vinegar, acid whey call "agra").
I use agra ( after making ricotta save some whey and store at room temperature per one day, after I store in fridge, this is my home metode).
At 80°C add agra, 1/4 cup for 5L circa or less, stir very gentle until ricotta start to rise and stop to stir;
Low the fire and wait for few minutes, turn off the fire and wait 10 minute.
Normali now you have ricotta!
If is not clear, please ask!