Home made ricotta cheese; what am I doing wrong?

I found a video online on how to make ricotta cheese. I had 2 quarters fresh raw milk brought to a rolling boil added salt and a half tablet of rennet dissolved in one quarter cup of cold water. Lowered the heat and mixed as per the instructions... and mixed... and mixed.. and it never curdled!!! I am still waiting it to curdle, I stopped mixing, will it ever curdle? should I add more rennet? what is wrong, I followed the instruction to a T~!