Inspectors found that although some areas of work had improved since children's services were transferred from Doncaster Council to Doncaster Children's Services Trust, services for children who need help and protection remain "inadequate", resulting in an overall rating of inadequate.
The decision to strip Doncaster of responsibility for children's social care was initially ordered by then Education Secretary Michael Gove in July 2013 as a result of serious child protection failings at the local authority dating back to 2009.
The independent trust, which is responsible for all children's services except services for disabled children, the virtual school for children in care and the early help service, launched in October 2014.
Ofsted's report states that services for looked-after children, as well as leadership, management and governance of services, have improved since 2012 when, under the control of the local authority, all services were deemed to be inadequate. However a series of weaknesses were identified.
The early help partnership offer, which is co-ordinated and led by Doncaster Council, was found to be "fragmented", with too many children and young people whose needs could be met at a lower level are referred for statutory services.
"In a small number of cases, inspectors saw serious weaknesses, including delays in holding strategy discussions, potentially leaving some children at risk of harm.
"The quality of assessments is not consistently good and for some children plans are not clear enough about the areas of concern or what actions are needed to be undertaken to reduce risk."
Meanwhile managerial oversight and supervision was found to "not always be sufficiently robust enough to address the variability of social work practice".
"The circumstances of some children and young people do not improve quickly enough once risk is identified, and they are left in neglectful or risky circumstances for too long," the report states.
"Partnership working with the police needs to be better co-ordinated to ensure that child protection investigations are timely and robust and consider all areas of risk."
Chief executive of Doncaster Children's Services Trust, Paul Moffat, said the inadequate rating comes as no surprise due to the level of change needed, adding that improvements are "well under way".
“We welcome the findings of today’s Ofsted report, which shows we are heading in the right direction on our rigorous improvement journey to transform services for children, young people and families.
“During the trust’s first year our main objectives have been to stabilise and improve existing services and to develop more effective ways to solve long-standing problems. We know that there is still a lot to do and the inspectors confirmed the challenges we had already identified.
"We are making strong progress against our improvement plan and we are on track to reach the targets which were set for us by the council and the government.”
The trust has been set a target of achieving a "good" or better rating from Ofsted by October 2017.
Earlier this year Prime Minister David Cameron announced that more local authority children’s services departments will be taken out of council control in order to tackle failure.