Reading Council's children's services were handed Ofsted's lowest rating in August 2016 after inspectors found children being left too long in "situations of unknown and acute risk".
A monitoring visit conducted earlier this month found that progress "remains too slow", with efforts so far to implement the improvement plan for the local authority deemed to be "too process orientated".
Inspectors highlighted the fact that implementation of the improvement plan is being led by an interim member of staff who is providing cover to the head of transformation and improvement following the resignation of the permanent postholder.
They added that work to stabilise the workforce has been "ineffective" with no progress made since the last monitoring visit.
"The majority of senior, middle and frontline managers remain temporary workers and the percentage of permanently employed social workers has declined slightly," a letter to Reading Council outlining the findings states.
"Instability at senior management level has increased with the chair of the local safeguarding children board, an assistant director and the head of transformation and improvement recently resigning.
"The local authority has very recently introduced some measures which are intended to address long-standing weaknesses in services for children it looks after, but these services are yet to show any sustained improvement."
Inspectors said that only slightly more than half of children on child in need plans are visited within the timescales stated in their plans and nearly a third have no plan at all.
Meanwhile, more than a quarter of referrals to the multi-agency safeguarding hub in 2016/17 were re-referrals, and a significant minority of children on child protection plans have been the subjects of plans before.
"This indicates that partner agencies remain uncertain about referral thresholds, and that statutory social work with many children at risk is still not effective in reducing serious concerns about their safety and wellbeing," the letter states.
"Significant delays and drift remain for children who are in the public law outline, where high-level concerns about them could lead to the onset of care proceedings."
However, inspectors did say that performance management has developed since the last visit - with monthly performance surgeries recently introduced.
Meanwhile, caseloads remain manageable for the majority of social workers, and "well-thought-out" measures have recently been, or are about to be, introduced to address long-standing practice weaknesses.
These include establishing an "access to resource team" and a dedicated court team which has started to lead on the preparation of evidence for children's cases in care proceedings.
A spokesman for Reading Council said: "The council accepts that the pace and scale of improvement has been slower than anticipated, due to a number of circumstances.
"However, we are now moving forward with all aspects of the council's learning and improvement plan, which has recently been revised to reflect the improvements made so far and to focus on the challenges that lie ahead.
"There are a number of positive findings in the Ofsted letter including: internal case audits being more focused on the most important areas for children; more effective quality assurance and performance management; social workers working hard to build trusting relationships with looked-after children; improved transition arrangements for young people aged 16-18.
The Council accepts the concerns raised with regards to pace in the monitoring report and recognises there is still much to be done."