A third of critically ill children had to wait more than three hours for a specialist ambulance to take them to intensive care units last year, figures have shown.
Transfers from district hospitals in mid, south and west Wales are made by a team in Bristol through Wales and West Acute Transport for Children (Watch).
It has a target of three hours to get to the patient's bedside.
But the Welsh NHS said there were no formal concerns from health boards.
Bristol NHS Foundation Trust, which runs the service, said it was proud to provide a "high quality intensive care transport service".
The Watch team made 316 transfers in 2017 but an audit found 31.6% of those took longer than the three-hour target to get to the patient's bedside - an increase from 28.1% in 2016.
Similar specialist teams across the UK performed better on average where 11.9% of call-outs took more than three hours, down from 13.7% the year before.
The creation of Watch was controversial as the team covers a large geographical area with 22 hospitals across south, mid and west Wales, as well as south west England from Cornwall to Wiltshire.
It was formed in 2015, replacing a Cardiff-based team which served south, mid and west Wales.
In February, several parents who used Watch told the BBC's Wales Live programme about the long waits their children had experienced.
A spokeswoman for Welsh Health Specialised Services Committee (WHSSC), which commissions the service on behalf of Welsh health boards, said: "There have been no formal concerns raised by health boards in Wales to WHSSC around the current service.
"Watch has recently been included in a UK-wide peer review of paediatric transport services and WHSSC is awaiting the outcome."
A University Hospitals Bristol NHS Foundation Trust spokeswoman said: "Our dedicated Watch team continue to provide a specialist service, 24 hours a day, seven days a week, accepting referrals and transfers of critically ill children to an appropriate paediatric intensive care unit.
"Accepting referrals from hospitals, stretching from Aberystwyth in the west of Wales down to Truro in Cornwall... mean that some retrievals can take longer the national target."