Get a psychological assessment done by a licensed Clinical Psychologist. A detailed history concerning her concentration span, maturity in comparison to age and academics will definitely help in reaching to a diagnosis, following which a specific treatment plan would be made.

Dear parent,  please immediately consult a Special Educator,  to rule out dyslexia or learning disability in your child.

Hi. From what you have mentioned, the thing that comes out is about her ability to learn alphabets. But to come to a conclusion about the reason, more information is required like her developmental milestones, her ability to perform other tasks which children her age can do. You can seek a paediatrician or a child psychitaric consultation for her. Depending on her other symptoms, her IQ test might be needed to assess her intellectual ability.