You've definitely not failed her.
You've noticed that she might need some support and you've bothered to come on here and ask about it.
That sounds like a good, motivated Mum who has her best interests at heart
You'll need to contact your HV and say that you've done the 6 month Ages & Stages and she's scoring in the Blavk in a couple of areas and that you need to see her.
I would also fill in the 6 month Social & Emotional Ages & Stages and take both of the completed forms with you when you see the HV.
Seeing both firms together should give your HV a better understanding of what's going on.
Hopefully she'll refer you to a Paediatrician. The very least I'd want is a referral to physio and a hearing test.
If the HV waves to adopt the "wait and see" approach, I would ask her for the hearing test and the physio referral anyway but also make sure she books the next appointment with you then and there if you can.
The next Ages & Stages is the 8 month one so if she does want to see her again before referring to a Paediatrician maybe ask her to see your DD again the week she turns 8 months.
You might want to ask the HV for a referral to Portage too although in some areas you can self refer 