Learning to Read
When I did my teacher training my lecturer said that a child learns to read by reading. We learn to ride a bike by riding it; in other words practice. This is true of many children BUT there are also children that need more, much more.
Children who can only focus on 2 or 3 letters at a time are not going to recognise words with 4 or more letters. A child can sound out a word many times and still not recognise the word when they come to it the next time. Children with this problem need to have very good phonics skills.
The child's teacher needs to be told about this problem. Schools can only go so far. Your child will need you to work with him/her at home. Before school often works better than after school when they are tired.