West Midlands Police has a lack of understanding of the extent of child sexual exploitation in the area it serves, a report has found.
HM Inspectorate of Constabulary looked at the force's child protection work and found heavy work loads and a "weak" response to complex or prolonged cases.
But those managing investigations were "committed to child protection" and staff were knowledgeable and dedicated.
Asst Ch Con Carl Foulkes said the unit had recently "doubled in strength".
He also said the inspection came just two days after new arrangements had been introduced.
Inspectors said they had found areas of concern they wanted addressing immediately and requested an action plan within six weeks which would set out how to utilise their recommendations.
'Engaged in the fight'
Among the positive areas the inspection revealed were the the good relationships with partner agencies and the introduction of mandatory training programmes for officers and staff.
HM Inspector of Constabulary Dru Sharpling said: "I am pleased to see the clear commitment to protecting children shown by both the leaders and the dedicated staff within West Midlands Police.
"Despite this, our inspection found areas of concern that meant children were not receiving the service they deserve.
"In particular the force needs to improve both its approach to the more complex child protection cases and a better understanding of the extent of child sexual exploitation in the West Midlands."
The June inspection was part of a rolling programme of child protection inspections of all police forces in England and Wales, a spokesman said.
Mr Foulkes said he was "passionate" about providing the best child protection service but it was "unfortunate" the inspection took place two days after new arrangements had been introduced.
"I want West Midlands Police to be the best in country - dealing with vulnerable children with professionalism and compassion," he said.
"Our Public Protection Unit has been doubled in strength to some 800 officers and staff. That means around 10% of the entire force are engaged in the fight.
"That displays our level of commitment."