An international treaty designed to ensure the swift return of children abducted abroad by a parent needs to be implemented faster, researchers say.
A study suggests the proportion of returns under the Hague Convention on child abduction fell between 2003 and 2008 and that cases took longer.
Cardiff University Professor Nigel Lowe says his study sends "warning signs" about the treaty's overall performance.
The body overseeing the treaty says it relies on members to operate it well.
The Cardiff study compared figures from 2008 with those from five years earlier and found there was a 45% increase in the number of applications to get children back, that the proportion of returns had declined and cases took longer to resolve.
"I'm particularly concerned with the time which I think is the essence of Hague cases," said Professor Lowe, describing himself as a "big fan" of the convention.
"The finding that they are taking longer to be dealt with for me is a worrying one and one that I wish to see addressed."
Constitutional v International law
The Hague Convention requires an abducted child to be sent back quickly to where they usually live, which is viewed as the best country to decide on matters such as custody and access.
A judgement on whether or not to return a child should ideally be reached within six weeks of court proceedings starting.
There are some exemptions - for example where there's a "grave risk" that a child would be exposed to "physical or psychological harm". Yet cases can take years to resolve with some never resolved at all.
One man who's been trying to get his daughter returned from Mexico for almost two and a half years told Radio 4's Face the Facts an "amparo" had held things up. It is a part of Mexican law that recognises a citizen's constitutional human rights.
"They told me they are going to hear this amparo in July last year, then they said they are going to push it back to August, then September. So until they decide about the amparo then nothing happens," the man said.
Mexico is a fellow signatory to the Hague Convention with the UK. Its Deputy Ambassador in London, Alejandro Estivil, said its supreme court had clearly resolved that constitutional law took precedence over international matters but insisted the country took its obligations seriously.
"I can guarantee that Mexican authorities are trying in every case to comply with the Hague Convention and be as swift as possible," he said.
"I think one has to acknowledge that the convention is not perfect and it isn't implemented perfectly everywhere", said Professor Louise Ellen Teitz, First Secretary of the Hague Conference on Private International Law and who is responsible for the treaty. "There are more cases and fewer resources."
Professor Teitz said the convention relied on its members to operate it well.
There are around 200 legal jurisdictions in the world. Only 87 of them are signatories to the Hague Convention, with no penalties for those that do not follow the rules.
The process depends on international diplomacy to encourage poorly performing countries to improve.
"We are very careful never to name names or to seek to criticise or stigmatise", said Lord Justice Thorpe, the head of international family justice for England and Wales.
"The whole process depends on consensus so you hope always that you can uplift some country that's not performing so well, by either example or by direct aid."
But Lady Catherine Meyer - whose two sons were detained overseas by her ex-husband 10 years ago and who now runs the charity Parents and Abducted Children Together - said the time for diplomacy alone was over.
She said she would like to see countries named and shamed and thrown out of the convention if they do not stick to it.
"If we go pussyfooting like this all the time, nothing is ever going to change. We need to be much more firm.
"The Hague Convention is better than nothing but it is absolutely not fool proof and some countries do not abide by it the way they should."
You can listen Face the Facts: The Stolen Families on BBC Radio 4 at 21:00 BST on Sunday 22 July and on BBC iPlayer.