I recently used Straus milk for a class on feta style cheese. I have never had any trouble with that milk. But... The milk was dated 12 days out from the day I purchased it. It did not set. I know it was not the rennet or the starter bacteria, or any sanitizer in the equipment. I brought out vinegar, and we made sort of a ricotta instead with one batch, and the other we left to coagulate enough that we could scoop it after about 1.5 hours.
In discussion with Gianaclis Caldwell (author of "Mastering Artisan Cheesemaking"), she gave some possible causes:
1. Fall and winter milk contains more plasmin and other enzymes associated with plasmin. Plasmin breaks down proteins. In the milk, it breaks down the casein. If the fresh milk from the fall and winter has problems coagulating, and you rule out the rennet and pH, then it is likely this problem.
2. Milk older than a few days is going to suffer some protein damage from cold storage bacteria not destroyed during pasteurization, the psychrotrophs. It won't be dangerous, most likely, but is harder to get a good coagulation from.
A further response from her indicated that just using more rennet is not going to make much difference...
Straus milk is relatively standardized and combined from several dairies. So it does not seem like it would be just a "fall and winter" problem, but could be....
I checked with Straus about their pasteurization process, and they use 173-175 degrees F, for 15 seconds. Perhaps it was hot enough to denature some of the proteins.
I am thinking that it is milk a few days past when I should have bought it to use in class, or that there may have been some less than optimal storage along the way.
My plan is to buy the milk on the same day it is delivered to the store, and to make a test batch at home first. I will test starting pH and if it shows elevation about 6.65 then I will use extra starter and extra rennet due to the milk showing signs of being "seasonal".