Better support services are needed to prevent adoptions breaking down, warns the children's commissioner for Wales.
Keith Towler said adoptive parents were often being "left to their own devices" when an adoption starts to fail.
It is estimated that as many as one in three adoptions may fail, often when adopted children hit their teens, BBC Radio's Eye on Wales reports.
However exact figures are unclear as data on such breakdowns is not centrally gathered.
Experts in the field are calling on the Welsh Assembly Government to start recording and researching the true number of failed adoptions and to invest in more support and therapy for families.
A recent report from Adoption UK, a charity for adoptive families, warned that care professionals are not learning the lessons from the "unacceptable" number of failed adoptions, due to this absence of data.
The report makes a number of recommendations for change.
Ann Bell, development manager for Adoption UK in Wales, tells Sunday's Radio Wales programme: "About a third of adoptions will go very smoothly, about a third of adoptions the families will need some kind of support… and then possibly up to a third, over the whole lifetime of the adoption, we think may break down."
'Working in the dark'
Ms Bell agreed that agencies were "working in the dark" over the exact rate of breakdowns.
She said: "The whole premise of adoption is that once the adoption order has been granted, that family is exactly the same as any other family… and because of that, there is no ongoing surveillance, supervision, recording.
"We need a proper piece of longitudinal research. That's really the only way we could get figures for both before the adoption order is granted, and also those breakdowns which we know happen quite often when children reach teenage years."
Mr Towler said that while much support was in place for looked-after and fostered children and families, access to local authority services and assistance from the Child and Adolescent Mental Health Services (CAMHS) for adoptive children can be patchy at best.
He said: "The thing that's struck me more than anything… is the extent to which adoptive parents feel very much left on their own, to their own devices, and that really is something that has to be put right, I think.
"We seem to be in a situation where there's lots of state intervention through assessment, making sure care plans are in place, then at the point of adoption then everything eases off, the adoption placement is made, and the impression I get is that at that point, those families then fend for themselves."
The Welsh Assembly Government said it was considering the findings and recommendations of Adoption UK's report.
It is consulting on a package of measures to strengthen the assessment, placement and care planning for looked-after children.