I assess for DSA purposes.

I enjoyed your text on automaticity. The driving analogy made it so clear to cascade to students.

Just one query: Why would a student know the word, letter, object, etc., but not be able to get it out?

The short answer to your question is that this is a problem with word retrieval, which is a rather complex skill. Here is a good piece that describes it.

Naming speed has been associated with a variety of reading skills, and in particular, it has been found to be closely related to sight word recognition, reading rate, and orthographic skill (i.e., understanding and facility with letter patterns and spelling rules).

I’ve got more information here.