Devon and Cornwall police have been told to produce an action plan on improving their child protection work.
A inspection has found several good points - but also a lack of understanding of the extent of child sexual exploitation.
The report by HM Inspectorate of Constabulary says the force "has much more to do to demonstrate that it is able to effectively identify and safeguard children at risk" of sexual exploitation.
It also says earlier intervention is needed in response to some children who go missing from home regularly.
It said there was an inconsistent response to child sexual exploitation across the force area, and sometimes investigations were undertaken by insufficiently skilled staff.
However the report does praise the staff who manage child abuse investigations, good multi-agency work to protect vulnerable children, and a clear commitment to improve child protection services.
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