Store bought milks are a breed all of their own. You don't state whether it is UHT or fresh milk. I am assuming it is fresh milk.
Because of the processes that store milk goes through, there is the thought that the addition of calcium helps the milk set and gives a better curd. Personally I haven't tried that, so someone else is going to have to comment on that.
I have though tried adding more rennent when I got almost no curd, and for me it was not successful.
Leaving longer to set, I feel, can over over incubate the cultures and milk and the cheese flavours are compromised. If enough curd has formed to work with, then I go ahead and do the best I can. If not, then it is a lesson learnt.
These are just my experiences.
