There are three main elements that determine the amount of noise transmited through floors, the floor covering, the composition and thickness of the concrete slab and the behaviour of the residents.
If you were to say that you never complained about the previous occupants because their behaviour (maybe not wearing shoes indoors) meant there was rarely any problem, then you wouldn’t have to justify why you are only now complaining.
That’s not the issue but it gets you past the “why now?” question and into the nitty gritty of the problem.