Keith Towler said he was increasingly frustrated at the lack of progress in the support services for such children.
An independent "voice" for children was suggested 14 years ago after an inquiry into abuse in north Wales care homes.
The Welsh government denied Mr Towler's claims, saying it was committed to empowering children to speak out.
Latest figures show there are nearly 6,000 children in care in Wales.
And every one should have access to an advocate, someone who offers them one-to-one support and can represent their views.
But Mr Towler said the reality was that services were patchy and inconsistent across Wales, with an "apparent lack of necessary drive and determination" from ministers.
He has spoken to 384 young people in care for his latest report and found more than half of them did not know who their advocate was.
"I can't deny that there has been some progress but it has been patchy and it has been slow," he said.
"There have been too many excuses for why change has not happened more quickly and, in the meantime, the situation for children and young people remains much the same."
Mr Towler said recent allegations about historical child abuse shows the need to get advocacy services right.
"Advocacy enables us to create a climate where we listen to children and young people, a culture where we can better protect our children. In short, advocacy safeguards children and young people," he said.
Mr Towler has called for a nationally commissioned model to bring consistency across Wales.
The Welsh government said it has improved the system.
"We do not agree with the conclusions reached by the Children's Commissioner for Wales," said a spokesperson.
"We remain absolutely committed to keeping children free from abuse and to empower them and others to raise concerns that ensure perpetrators are identified, investigated and subject to the full weight of the justice system.
"Wales was the first country to appoint a children's commissioner to speak up for children and to provide scrutiny and challenge to everyone with responsibility for safeguarding children."
Children in care in Wales still do not have the support they need and are being denied the right to an advocate, the children's commissioner claims.