First Bus has said it is "deeply sorry" after repeatedly leaving a 13-year-old disabled girl at the roadside.
Kath Gibbons said her daughter Lily, who has cerebral palsy and is confined to a wheelchair, had two "absolute nightmares" while waiting for buses.
First blamed a breakdown in communications.
Mrs Gibbons said two weeks ago Lily and her carer Jodie Bidgood had to wait four hours in Helston for a bus with a disabled access.
And last Saturday, despite twice being assured buses Lily wanted to catch had disabled access, neither did.
"It is an appalling service provided for disabled passengers," Mrs Gibbons said.
"Lily needs more independence so her carer calls to take her out so she can pick up some life skills."
Lily wanted to visit friends in Falmouth but had to wait for hours for a bus with the right access. "They went to Penzance in the end," her mother said.
Last weekend, Mrs Gibbons rang in advance on Friday. "They said if I got Lily to Helston by 9.45am the bus would have a disabled access.
"But when it turned up it had no disabled access at all so I drove them to Falmouth. I couldn't believe it."
She then rang First again, and added: "They were quite apologetic and said they would make sure the 3.15 bus (from Falmouth) would have disabled access." It did not, so Mrs Gibbons had to drive back to Falmouth from her Porthleven home to pick them up.
"I phoned customer services and they said it was very frustrating."
She also said it was unfairly making Lily feel her disability was a stumbling block to getting around.
A company spokesman said: "We are in the process of updating our entire fleet to be low floor and easily accessible in time for the Disability Discrimination Act deadlines.
"We do have a policy of making reasonable adjustments for our disabled customers and where we are able to. We have investigated the complaint made by Mrs Gibbons and have found that a breakdown in communication on our part led to the failure to provide a low floor bus on Saturday.
"We are deeply sorry for this error and for the inconvenience this caused.
"We have reviewed the internal process that contributed to the error and will do everything possible to avoid similar mistakes being made in future."