www.Leeners.com is a popular and reputable cheese supply store so probably it isn't the rennet itself.
Another problem I had was on
this 2 US gallon batch. At the time I only had 1 US gallon stockpots so I made it in two pots, front one set great, back one didn't even aftr waiting ages. Same milk, same warming rate, same starter culture, same calcium chloride, same rennet, same whisking for 2 minutes each. I think what happened is in preparation for adding
my liquid rennet I diluted 50 drops in 1/2 cup of water and stirred but then let sit while milk continued to warm. I notice that my liquid is slightly brown-orange and when counting drops into water it sinks to bottom, so need to stir in water to dilute. I then trickled 1/4 cup into back problem gallon while stirring and then 1/4 cup into front stockpot. While sitting, my diluted rennet may have re-settled in the water to bottom somewhat and then the back problem stockpot only got the 1/4 cup off the top with low rennet concentration and the front stockpot got most of the rennet. Thus creating my problem where front pot set great and backone took ages.
Rennet is a very concentrated coagulant that needs to be diluted and stirred in throughly to make an even distribution. Is it possible in your batch that rennet was not thoroughly distributed or that not enough and thus while set, not a strong set? My directions say 20 drops per 1 US gallon and I'm already using 25 . . .