I have a Roomba 500 series, which would do its little dance going through every room in my house. When it's done, however, the dustbin is always left empty! (and the floor is still dirty)
I noted that the "agitator" part (the 2 revolving brushes) does work, so all these great robotshop guides won't do any good
I Also found this which is way too advanced for me.
I suspect that the vacuum motor isn't getting any juice, or jammed. Any ideas?