The variable that predicts nearly everything and that people ignore: distance from the glass.
Light falls off extremely fast as you move into a room. A spot two metres back from a window can be a small fraction of the light one metre back, and a fraction again of the windowsill. That is not a modest difference; it is the difference between growing and slowly declining.
So when someone says a plant is by the window, the useful question is how far, and the answer is usually further than they think.
What this means practically:
Move things closer before doing anything else. It is free and it is usually the whole fix.
Watch what is between the plant and the sky. A net curtain, a building opposite, a tree in leaf, a deep window reveal. The plant sees the sky, not the window: anything blocking the view of sky is taking light.
Clean the glass. Genuinely worth doing once a year and it makes a measurable difference.
Which way it faces matters most in winter. A south-facing window in July and the same window in December are different rooms as far as a plant is concerned, and this is why plants that were fine in summer decline from October.
A phone light meter app is not accurate in absolute terms but it is excellent for comparing two spots, which is all you need.