More than you ever wanted to know about coagulation:
https://www.cheesesociety.org/wp-content/uploads/2011/02/2011-Choice-of-Coagulant-Preparation-McCoy.pdfhttp://www.umb.no/statisk/nordost/laht_renneting_jurmala.pdfhttp://www.agriculturejournals.cz/publicFiles/50919.pdfhttps://www.uoguelph.ca/foodscience/book-page/enzymic-coagulation-milkThe info in these various articles / presentations is a bit confusing - more than one of them says that the ideal coagulation temperature is 30-32°C, and yet they all suggest that the coagulation activity increases up to at least 40°C or higher. Note also that they do not make clear whether there is a difference between the temperature response of chymosin-only (e.g., GMO produced - which is what several of the above seem to assume as the primary variety) or animal (chymosin + peptide), and they don't really mention (or if they did, I missed it) vegetable rennet, e.g. made from thistle or such.
Some other possible variables - 1) When heating the milk to the higher temperature before adding the rennet, did you heat evenly / stir continually / prevent scorching? This could make a difference. 2) Assuming that you diluted the rennet in non-chlorinated water, was there any difference in how long it was between preparing (diluting) the rennet and adding the rennet? I have no first hand experience, but I have read that it deteriorates relatively rapidly once diluted. (Can anyone confirm this?) 3) Have you calibrated your thermometer? Easiest way to calibrate is to use two data points: first a pot of boiling water (full boil, and take altitude into consideration as necessary, and second a glass of ice water with lots of ice; you should get around 100°C and 0°, respectively. If not, calibrate your thermometer if possible, or calculate the error and adjust for it. As a final check, use the calibrated thermometer to prepare water at 37° (98.6°F, body temperature), and compare the reading against an oral thermometer. 4) Are you making sure to give your thermometer sufficient time to stabilize before taking a reading?
To answer one of your questions - yes, some recipes add the rennet with no ripening time. This will affect the coagulation, but it still should coagulate, as I understand it. Certainly I've made many cheeses with minimal (10 minutes) or no ripening time, and the results have been successful.
If it were me, I think I'd do some experimenting before committing to a full batch of cheese - using your calibrated thermometer, carefully warm one cup of milk to 30°C and one cup to 42°C, making sure to heat gently / not scorch. Prepare a dilution of rennet, put exactly the same amount (maybe 5 ml) in each cup, and float an upside-down bottle cap on the surface of each one. Check to see how it responds over the next 10-30 minutes by pushing the bottle cap around - as it begins to gel, the cap will stop moving easily. If you have used the same milk, same diluted rennet, same amounts of both, you should be able to get a sense of the effect of temperature apart from any other variable.
