Wayne's correct, you don't NEED a PH meter, but it makes things WAY easier. If you're interested I have a used one I might be willing to part with. PM separately.
As Wayne said sour is too much acid or bad milk or other nasties in it that are souring the milk. Curdling milk is also known as souring the milk, you can use vinegar to sour milk and curdle it to make cheese. Also bitter is usually an indication of too much rennet.
As for the milk, milk's starting PH will ALWAYS be 6.6-6.9. The closer to 6.9 the fresher it is. If you need a PH meter you need one that goes to .01 not .1 which most do. You can always add more starter, to a point, what kind of starter are you using? If you're using direct set then milk with antibiotics in it will kill the bacteria in the starter, but this is rare as antibiotics are not supposed to be in there, by law I think. If you're using tap water the chlorine can do things bad as well.
I once waited 3 hours for the PH to drop on a Parm and it set like crazy, very nice break, but the curds by the time they were done cooking were hard. You said cook longer, usually you bring the milk up to temp add the starter wait until the PH reaches your target then you add rennet. Cooking implies you are heating your milk up to the next level. Anyway, what I would do if you get a PH meter is wait up to 3 hours for the PH to drop. If it hasn't then I would add rennet and call it a test batch. If you are using the same milk try a different brand. Trust me you wouldn't think that but all milk is not equal, even out of the same dairy.