The system weighs your bag and if over the allowance, will not accept it and automatically calls a human. There are humans circulating to help folks but, if it's peak time, there may not be enough humans to go round.
It's the kind of problem that will soon be fixed, once most of the pax using the self-check and bag machines are of the generation that grew up with computers and self service. So, only about 25 years to wait ...