Sa'ad, welcome to the forum. There's a lot of great info here -- though you might find some of it conflicts with the rest.
There are some here who say it's just not possible to get a good stretch out of the 30 minute mozzarella. There are others who say they have no trouble getting a nice, stretchy moz for pizza.
I believe the latter, but I've never been able to duplicate their success. Mine always stretches well out of the microwave, forms a nice, satiny sleek ball... but when it goes into the hot oven, it just sits there and gets grainy, almost like a goat cheese. (Still tastes pretty good, though.)
Anyway, my problems are not your problems.
There aren't too many variables to deal with in directly acidified mozzarella (compared to some other cheeses I can think of). Are you using a different milk, citric acid, rennet, or water than you did when you had success? It's hard to imagine a situation in which you get no reaction from the rennet. If it's old, it might not work as well, but should still work. Likewise, if your milk is ultrapasteurized or otherwise not good, you should get a weak, sludgy curd, but still get something. If you don't have enough acid, you won't be able to stretch the curd, but it'll still form a curd (unless you have WAAAAY too little). Possibly if you have heavily treated water, it could be destroying the rennet... but I've never heard of that happening. Just in case, I use filtered water, and I boil it in an open saucepan for a few minutes and then cool it before adding the rennet to it.
Your experience with the near-instantaneous curdling is the same as mine. When I have everything right, I stir for about 15 seconds, because I once went 30 seconds and the curd had already started to form, and I ended up breaking it back down. I thought I'd ruined it, but was able to scoop out the little broken curds with my hand, and then in the microwave they came back together.