Maggie Cork, who left Leicestershire Partnership trust in January and now works in the independent sector, spoke out as plans to ensure under-18s have access to age-specific facilities were dropped.
It was proposed as an amendment to the Mental Health Bill by the Lords but overturned using the government's Commons majority.
Ms Cork, managing director of the Huntercombe Group, said it was a 'mystery' why the problem remained.
Ms Cork.told HSJ: 'The question for me, having come from the NHS, where many provider mental health organisations struggle with a national dearth of children's beds, is whether commissioners are aware of the availability of specific specialty beds in the independent sector.
'Or perhaps the question should be, are they willing to commission such provision?
Andrew McCulloch, chief executive of the Mental Health Foundation, said while some commissioners do not understand the needs of young people with mental illnesses, they were often constrained by finances available to them.
He said: 'There's a lot of pressure to avoid placements in the private sector, even though it may be better for a child to be placed in a specific service.'
He said he was 'disgusted' the Lords' amendment had been overturned.
Health minister Rosie Winterton's pledge to come back with new proposals was 'an excuse for inaction' he added.