I was trying to make Paneer (a type of cheese). So I boiled once then added 2 table spoons of lemon juice. Milk should be separated out instantly from water but it's been 20 minutes.. And milk is still unchanged.
Milk proteins will coagulate at particular temperatures and Phs. You wrote that you used 2 Tbs of lemon juice but you didn't say how much milk you added that to. If you used too much milk then the mixture will not be acidic enough. Follow a recipe.
You also wrote that you boiled the milk once. I don't know if it is a language barrier issue but it sounds like you boiled the milk in the past. The milk needs to be at near boiling temperatures when you add the acid. The combination of heat and acid will cause the milk proteins to unravel (denature) and tangle up with each other (coagulate) which will result in the curd you are looking for.
If the milk has been handled well, that is if you didn't leave it out overnight, then it has likely not spoiled and you can try again by either adding more acid or bringing to a boil as necessary.
The milk could be adulterated, treated with detergent, urea, or made using milk powder.
Milk instantly curdles to form cottage cheese when lemon juice or vinegar is added.
Adulteration is rampant in India. Even if it's just diluted with water it should curdle to some extent; even skimmed milk curdles, although it would produce less cottage cheese. The separated water is whey.
If you're in the US, maybe it could be added corn syrup that could hinder curdling of milk?
I'm making paneer for the first time, too, and I was having a hard time getting it to curdle and googled why, and ended up here.
There is some question about the milk/acid ratio you are using. I was using Indian recipes, and none of them provided a precise ratio. Look up how to make cheese. All cheese making starts this way and regular cheese recipes have more precise measurements to start with. It's not exact and never will be, suffice to say, a measurement is just a starting point and knowing you might have to gradually add more acid is all the instruction you can hope to get.
Here's what I've learned so far: Don't boil the milk. Heat milk slowly enough so that it doesn't scald. It needs to get to at least 180°F/82°C It's okay to stir the milk while it's heating up to help prevent scalding.
Tip: Before you add the acid, stir the milk so that the milk is moving when you add the acid
**Don't stir the milk after you add the acid. Turn off the heat and wait up to 15 minutes for the curdle. *It doesn't happen immediately Different types of milk will take more or less time to curdle, but the curdling opportunity is complete at 15 minutes. If the curdle isn't complete, don't stir, and heat the milk again. The curdle may complete while heating, this instruction is as imprecise as the amount of acid. My tip is to take care to heat it correctly the first time! The milkiness of the water is cheese! The water (whey) should be clear-ish. It might be yellow-ish. Look at several YouTube videos on cheese making to see what clear-ish looks like. I say several because some are less clear than others.
Lastly, I suggest experimenting with smaller amounts of milk until you get a decent result. And try to remember that different types of milk behave differently. Different acids behave differently.
It's not complicated, but that isn't the same as easy.
The next thing you'll be fighting is the softness of the cheese. Lots of recipes use the word "soft" to talk about the texture, what they mean is, not rubbery. Not rubbery is what soft cheese is!
I had to laugh at all the times I accidentally curdled milk, and there I was trying to curdle it, and it wouldn't!
I was able to make a decent batch of Paneer, and you will too.