Open a window and look down. There is a channel under the frame with something in it that is no longer identifiable, packed into the corners and flat against the bottom. You have run a cloth along it before. The cloth went black and the track stayed full.

That is a geometry problem, not an effort problem. A window track is deep, narrow and square-cornered. A cloth wrapped round a finger is round and soft, so it can only ever contact the middle of the floor of the channel. It rides over the corners, and worse, it presses the debris into them. Every pass you have made has been compacting the problem while removing the top of it.

A thin, stiff bristle brush is the shape the job actually needs. It is narrow enough to sit down in the channel rather than across it, and stiff enough that the bristles do not fold when they meet the corner, so they get under what is packed there and lift it out instead of sliding over it. Twelve in a pack because the bristles do wear, and because the same shape solves a lot of other things: a shower rail, the seal round a hob, the gap beside a worktop, a car door channel.

Vacuum first, brush second, vacuum again. Brushing a dry packed track without lifting the loose material first just redistributes it.

Here is the limit. These are small brushes and they are not built to last forever. Used hard on grout or a rough track the bristles will splay within a few sessions, which is why the pack is twelve. Treat them as consumable and they are good value. Expect a lifetime tool and you will be disappointed.

They are also not much use on anything wide. This is a crevice brush. On a flat surface it contacts almost nothing and you would be better with a cloth.

And they do not clean by themselves. On heavy grease you still need a degreaser; the brush gets it into the corner, it does not dissolve anything.