Early speech expressive problems are frequently due to poor phonological awareness. Often students with early speech difficulties eventually acquire a 'dyslexia' diagnosis.
When the speech problems eventually clear up, it is assumed that the problem has disappeared. In fact, the underlying phonological difficulties can remain and manifest themselves as reading/spelling difficulties.
And it can be the other way round too. Pupils with poor articulation due to physical difficulties eg cerebral palsy affecting the throat and mouth can find it difficult to learn to spell and read. I think this might be partly due to not being able to have the constant monitoring feedback given by saying sounds oneself while looking at the grapheme.
Using the diagnostic tools in Fast ForWord we can identify the issues precisely.