Bus drivers are supposed to check the bus before leaving the depot including the door ramps and blue button to advise driver a wheelchair user is getting off next stop. It's not uncommon for the ramp to fail, or be deployed in front of street furniture or gutter making it inaccessible.
The button often doesn't work so I've learned to press both blue and red buttons so the driver will stop. If the pulls off before deploying the ramp I press red button repeatedly to stop the bus.
I've complained about this at the depot but nothing seems to change.