Brushes on my roomba don't seem to be spinning.

Try running your roomba with the dust bin removed to see if the brushes turn. Take the brushes out and make sure they're clean. You may need to replace the brushes if they're worn. Failing that, you can find instructions on how to take your roomba apart. Take out the brush assembly and open the screws on the side of it to expose the gears that drive the brushes. There will probably be a lot of gunk in there jamming them up. You might need to check the motor too.