I'm an adoptive parent not a foster carer but I would say keep pushing. I have had/ am having a similar experience with one of mine. It feels as if people spent decades failing to understand the ways in which complex trauma/ abuse / neglect could affect a child's long term development, or the importance of early attachment. That's changed massively, thank goodness. But it now makes me wonder if we have almost done too good a job, in that if a child is adopted or fostered everything is attributed, often rather vaguely, to 'attachment'. Whereas most of our children, in my view, are complex little mixtures who certainly have attachment issues to varying degrees but are also at high risk for developmental problems, drug and alcohol effects, inherited learning disabilities and mental health issues ... you need to work out what the mixture is for your DD and that may very well need the involvement of other professionals. I'd start with a paediatric referral and/or CAMHS.
Is there anything you're aware of with birth parents that could be an inherited issue, even if just a cery chaotic life - there's a perfectly sensible theory that in some families the inability to cope or parent effectively down the generations is about undiagnosed and potentially heritable developmental or learning issues.
Do you have any access to the Adoption Support Fund as a long term carer? They can do very comprehensive therapeutic assessments which might disentangle attachment from the other stuff. If it is all down to attachment, what specialist help are you getting? A 6 year old with disorganised attachment would be very challenging and need specialist help.
A useful question to ask can be 'What would you be considering as a diagnosis if this child wasn't adopted/LAC?'
You could also talk to the school about applying for an Education, Health and Care Plan if she is really struggling in school.
Do push for the referral to enuresis nurse if she is struggling in that department, they are very good generally.
Best of luck.