Have you asked her teacher how to help her at home..?
Also, if she is being called Dumbo, you should speak to her teacher about this. This is not acceptable. The school should be doing all that is possible to boost her self esteem..... not allowing this type of nasty behaviour from others.
I suggest you make an appointment to see her teacher. Express your concern about her learning. Ask them if they think there are any underlying problems that are hindering her learning (could she possibly be dyslexic? Ask them to test her.......). I would explain about the name calling and ask what they are going to do about that... and ask how you can best support your daughter at home.....

Also.... have her eyes been yesyed recently? What about her tracking? Some children havce tracking issues... so might be able to see but can't read very well as th have difficulty tracking the words. Unfortunately, in the UK, the behaioural optometrists who can give exercises to help are all private, I believe.... and not available on the NHS.
Good luck......