What kind of milk are you using?
I've found that even the plain (not ultra) pasteurized stuff doesn't work well for me.
I get the curd to form fairly well, but always have a problem getting it to hold together. I either end up with really crumbly stuff, or stuff the texture of cheddar...
That, and I hate throwing away ~3/4 of whey (since I can't really find much to do with it).
Sincerely, a frustrated mozz "maker"