Matt, just to chime in, I've spent part of today updating three webpages
on Coagulation, Rennets, and Coagulation Problems, still needs some work but better.
When Baby Chee asked if you used salt, I think he is asking if you added Calcium Chloride
which helps in coagulation of pasteurized homogenized store bought milk.
Hope those pages help and as zenith1 says, some more details would help. While lower pH from an active starter culture helps coagulation, it is not required, you should be able to coagulate plain milk with just rennet.
If your batch is ongoing as we speak, then you have two choice with it, either proceed to use it for a primarily acid coagulated type cheese such as Neufchatel
, or dilute and add more rennet.