Sorry for the very late response. I find it hard to believe it's contamination. Acid maybe (soap or sanitizer left in the container, something gone rancid inside, maybe it's your utensils or thermometer, the dish that you used for rennet or cultures?) but contamination would have to be pretty bad for this to happen. 5 minute of milk acidification should be enough for you to get curd after you add rennet. You hardly need to create bacteria for this enzyme to work. Adding the bacteria and acidifying for so long only helps the milk because you are growing an overwhelming competing bacteria that eats up all the nutrients of other contaminating bacteria and pathogens. Acid though is capable of changing the milk pH violently enough to keep the rennet from working. If that's not it, than maybe you just had a funky milk. This happens with lactation and seasonal changes, animal diet and medicine, etc. It has happened to me before and to many others on this forum, one batch of milk that just doesn't cooperate.
Are you using a single or double strength rennet? Have you switched rennet to a new bottle size or a different brand lately?
As for the stacking - a cheese's weight is too much for a Camembert to handle. The french discs are about 35g-80g. Your fresh watery cheese is probably about 300-400g. It may expel the whey too fast before it is done working its magic. Just let it drip until it's done. I guarantee it will give you a better cheese.
