This stumped me for a while. You take out the big brush to clean the hairs wound around it. Then you can’t fit it back in! It somehow grew longer.
What happens is that hair or string is wound around one side of the brush where the fittings are–the side where you can pull off a small nut with your fingers. Pull it off the brush, then use scissors or tweezers to pry off the tightly wound hairs. They can be so tight that they look like they are part of the brush roller–but they’re not. And they push the nut out enough that you can’t fit the brush back into the housing.
I’ve had a couple of robot vacuums before, but this one actually works! And works very well.