Mark, I buy whole past & homogenized cow's milk from whoever is cheapest here in Houston TX. Sometimes Kroger, sometimes HEB, sometimes Randalls (Safeway I believe) cheapest brands and sometimes SamsClub, and in general never have a poor curd set unless I do something dumb.
Rennet times are guidelines only and depends on several factors, plus curds when freshly cut are very fragile, and even when cooking.
Can you tell us a little more about your method, ie what temp milk when add starter, what starter culture, how add, do you add CaCl2, how long before adding rennet, what rennet type and how stored, how add rennet, time to cut, healing time after cut etc? Just in case it's your method or other ingredients rather than the milk.
John