Heather Hawkes says her elderly mother Stella has been disappointed by Dial-a-Ride
Disabled and elderly people in London are finding it more difficult to book a journey with Dial-a-Ride.
Figures obtained by the Liberal Democrats show the number of rejected journeys has risen to 150,000 a year.
The problems have arisen since Transport for London (TfL) introduced a new centralised booking system in 2008.
A TfL spokesman said demand for the free service had increased. But he said it still managed to accommodate 91 out of 100 requests.
But the Liberal Democrats say the figure show 400 users a day are failing to get the journeys they want.
Heather Hawkes, a daughter of a Dial-a-Ride member, said: "I feel let down and I feel sad for mum because I know she looks forward to going to this club once a month.
"When she phones Dial-a-Ride they say, they'll let you know, or it is too far, or it doesn't have the buses.
"I feel sad because she only uses it once a month."
A TfL spokesman said: "Dial-a-Ride services have been getting better, and that - coupled with the fact that it has been free since January 2008 - means we've seen an increase in demand.
"We know refusals are disappointing, but when members can be flexible on timings, we can almost always find a way to accommodate their request and so far this year we have managed to accommodate 91 out of 100 trip requests.
"If members are unable to get the journey they want through Dial-a-Ride, there are alternatives available such as the Taxicard and Capital Call schemes".
But a London Liberal Democrats spokesman said: "It is time that [London Mayor] Boris Johnson and TfL stopped making excuses and started to address this serious situation."
Dial-a-Ride users make 1.2 million trips each year, on 350 minibuses.