Is it just spelling ?
Does she have any other common symptoms like:
www.nhs.uk/Conditions/Dyslexia/Pages/Symptoms.aspx
there are various other list out there with. Is there any family history of dyslexia?
Is she guessing words when reading ? Was she taught via mixed method or whole word?
Might it not be dyslexia but just poor spelling?
How are spellings taught by the school - are they taught or are they just given spelling tests every week ? Are they ever corrected in written work or is she making same mistakes again reinforcing incorrect spellings? Is it just spelling test that are the issue or is it spelling in her actual work - are there other issues with written work ?
Have you spoken directly with the SENCO - asked them why - what they have done / are doing to help her? What they view the problems as?
www.nhs.uk/Conditions/Dyslexia/Pages/Diagnosis.aspx This suggest You can challenge the decision and request your child is formally assessed through the statutory assessment process by contacting the special needs department of your LEA directly. Which suggest you can force them - but whether you'd actual get any useful help I couldn't tell you.
I know people who've shelled out for private assessments only to find it changes nothing at the school or worse lowers expectations of their DC.
Is your DC in the position mine was for years of everyone suspecting something is wrong but not struggling enough to merit help?
I suspect you need to ask for more information from the school if possible.
In mean time apple and pears good place to start sorting the spelling issues out.