A monitoring visit to assess progress at Kirklees Council - which was rated "inadequate" by inspectors in November 2016 - found that although there were pockets of discrete improvement in the quality of practice, children are still being left at risk.
Earlier this month, children's social workers at the council took part in a 48-hour strike organised by Unison, saying they were unhappy with pay, heavy workloads and bullying.
Inspectors said the pace of change is being hindered by workforce instability and high social work caseloads.
"Not all actions taken by senior managers to tackle drift and delay for children have been effective, and some children have been left in risky situations for too long," states a letter outlining the findings of the visit.
"Social workers are not able to complete all the tasks needed to support children and families effectively, because their caseloads are too high.
"A high turnover of staff is impacting adversely on continuity for children. The local authority is doing all that it can to recruit experienced and high-quality staff, but has not secured a stable and experienced workforce."
Inspectors said that, despite the issues, staff observed during the visit were child-focused and motivated to improve children's experiences.
However, they added that for some staff, morale is low.
It emerged yesterday that the DfE is looking for a new commissioner to help improve the quality of children's services in Kirklees.
Eleanor Brazil was appointed to the post last November, and submitted a report to ministers on her recommendations in March.
In June, it emerged that Kirklees had begun receiving expert support from Leeds City Council, amid speculation that the local authority was set to take over running the department.
However, the DfE is now looking to appoint a new commissioner to start work in October. They will be asked to "drive improvement and work with the council to oversee the establishment of a preferred delivery mode".