Please put your hand out in good time for the driver to see you. Don't assume the bus will automatically stop.
The drivers will scan the stops as they approach the stop for possible passengers but as they get closer and see no signal they will commit to moving the bus on. This means they may be looking in the road ahead or in their mirrors and may not see you.
At this stage it will be very dangerous to just stop the bus.
The onus is on the passenger to hail to bus, please keep an eye out for the bus.