A DfE commissioned evaluation study found that although the Adoption Support Fund (ASF) has resulted in improved outcomes for families, there have been a number of issues.
A total of £52m has been spent through the fund since it launched in May 2015, helping 22,000 children. Support can include cognitive therapy, play and music therapy, and parenting skills training.
But a cap limiting how much could be claimed at £5,000 per child - known as the fair access limit - was introduced in October 2015 due to unexpected levels of demand.
The report found that in the two years since the fund was set up there had been issues with a lack of suitable support services within the sector.
It said this had led to "excessive workload increases" for existing post-adoption support teams, adding that there was "insufficient capacity to meet demand".
The report said the lack of capacity in the system is yet to be addressed due to a lack of confidence in the long-term future of the initiative.
It notes that, following the launch of the fund, the market for independent post-adoption support services did expand in response to the increased funding available and the limits on the capacity of local authority adoption support services.
However, it adds that the independent sector is "not yet sufficiently developed to meet the rapid and substantial increase in demand".
"Key challenges to growth of local markets to meet the demand are lack of trained therapists in the ASF approved therapies and the capacity of the independent providers to fund and provide the necessary supervision required to practice effectively," the report states.
"In addition local authority adoption support professionals raised quality concerns about the market and this is exacerbated by the stretched capacity of independent providers struggling to meet the sudden demand."
The report said that there is yet to be a response to the capacity issues because of "lack of confidence in the future of the ASF" caused by the introduction of the fair access limit, no guarantees that it will continue past the current parliament, and the way in which regionalisation of the adoption system (via regional adoption agencies) could affect local areas.
However, despite the concerns about capacity, parents who had been allocated ASF funding reported high levels of satisfaction. A total of 74 per cent said they were satisfied with the process, 73 per cent said they were satisfied with the identification of needs, and 72 per cent said they were satisfied with the consideration of their view and preferences.
Meanwhile, local authority staff and therapeutic service providers overwhelmingly agreed that quality of provision had improved since the launch of the ASF, and families viewed the ASF-funded support as "appropriate and generally of high quality".
And despite a short time period between baseline and follow-up surveys, children receiving support through the ASF were found to show "small but significant changes" in improved behaviour and mental health, and decreases in aggressive behaviour.
The vast majority of parents (84 per cent) said they believed that the ASF had helped their child. Yet despite positive changes on most indicators, children's needs were found to remain "extremely high and complex" at the follow-up survey stage.
As part of a four-year adoption strategy, published in March 2016, the Adoption Support Fund increased from £21m in 2016/17 to £28m in 2017/18, with further increases in every year of the current parliament.
Children's minister Robert Goodwill said: "Every parent wants their child to grow up feeling loved and understood, and anyone with the commitment and compassion to adopt a child should have the backing of a strong support network.
"We know that caring for these young people, particularly those with more complex needs, can be a struggle at times.
"With the right therapeutic support, children and families will be able to embrace the new life ahead of them, and I'm delighted that the adoption support fund has supported so many thousands of people already, as part of our plan for a fairer society."
Goodwill also announced that a further £5m will be invested in a number of innovative projects across the country, designed to improve families' experiences of adoption, through the DfE's Practice and Improvement Fund.
Projects that will receive funding include three regional adoption agencies, which will develop new centres of excellence for adoption support.
The new centres of excellence, based in Hertfordshire, North East Lincolnshire, and Stockport will bring together social care, health and education experts to "provide a co-ordinated assessment and support offer for families".