1 ltr Milk is not curdling even after squeezing 2 lemons; What could possibly wrong?
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.