When I do the simple soft cheese with just lemon juice I find you have to get the milk just shy of boiling for it to curdle - my guess therefore would be that the milk isn't hot enough - if its not curdling try leaving it on the heat for a bit longer. Ricotta is often the same, you think its never going to go, but eventually it drops out if you just keep heating.
I don't do this very often though as I find that the heat and harsh acid produces a comparatively bland cheese - it certainly doesn't age as nicely as a rennet cheese. Our standard soft cheese uses the moorlands cheese ma400 starter and either animal or veggy rennet and the same combo works for hard cheese as well so reduces the amount of different cultures I have to keep. I make 2-3kg of goats cheese a week so feel free to ask more questions!