Children in parts of Wales are waiting an average of two years for an autism assessment despite a target of six months, figures have shown.
Research by BBC Wales' Wales Live programme has revealed staffing issues have also impacted on waiting times.
One mother said it took five years and £6,000 to get her son an assessment and his diagnosis recognised.
The Welsh Government said the roll-out of a national integrated autism plan was continuing.
A spokesman said there were also plans to issue statutory guidance to councils and health boards requiring them to plan and deliver effective autism services.
The Welsh Government has set a target of 26 weeks from referral to diagnosis.
But a freedom of information request to local health boards revealed the average wait is 107 weeks and six days in Hywel Dda area, which covers Carmarthenshire, Ceredigion and Pembrokeshire.
And it takes an average of 39 weeks in parts of Betsi Cadwaladr area, which covers north Wales, for a child to be seen.
It has said that historic vacancies within the neurodevelopmental teams have impacted on the ability to deliver the service.
'Finding out what was wrong with our son'
Clare Norton, from Milford Haven, Pembrokeshire, said her son, Ethan, was first referred for an autism assessment in 2012, when he was four.
But, after a five-year wait, she eventually paid around £2,000 for a private assessment.
He was diagnosed with a specific form of autism called PDA – pathological demand avoidance.
But Ms Norton said she then had to fight to get the diagnosis accepted by the local authority and health board because PDA was not universally recognised.
"It's on the autism spectrum although it's a postcode lottery in the UK as to which local authority or health board will accept it," she said.
"On a good day we can get lots out of him, on a bad day we can go from happy to aggressive, attacking us within seconds."
Ms Norton said she had spent around £6,000 on various assessments for Ethan, nine, as well as legal fees to get his diagnoses recognised.
"It's about finding out what is wrong with our son to help him," she said.
The Association of Educational Psychologists (AEP), the body which represents professionals involved in the assessments, has also raised concerns about the long delays in diagnosis, as well as a "postcode lottery" of support available in Wales.
AEP member Andrea Higgins, who trains educational psychologists at Cardiff University, said: "It is unacceptable for a parent to have to wait more than six months in my view.
"They become more resistant to interventions and support."
She added that there has been a fall in the numbers of educational psychologists in Wales in recent years.
"Investment in the workforce is what will make a difference," she said.
Wales Live sent a questionnaire to parents of children with Autism Spectrum Disorder (ASD) and received 200 responses.
One said their daughter had experienced depression and suicidal thoughts by the time she had been diagnosed, another talked of "seven years of pure hell trying to get help and a diagnosis".
A Welsh Government spokesman said: "We are rolling out a new all age National Integrated Autism Service which will address gaps in current provision and will align with the Together for Children and Young People neurodevelopmental work stream.
"The service is now open in Cardiff and Vale, Gwent, Powys and will open shortly in Cwm Taf. The remaining regions will open during 2018.
We will underpin these commitments by issuing statutory guidance – subject to the outcome of a consultation – on autism this assembly term which will re-enforce the requirements placed on local authorities and health boards to plan and deliver effective autism services."