The written form of a language is the inner speech of the language. For instance, you usually think what you want to express before you write it. It is necessary to have a good grasp of the language to be able to write it well. Quite a few ASL students often have rather poor writing or expression skills, especially when it comes to reading and writing in English.
Language researchers have found that, rather than just poor comprehension or learning skills, this is really rather an issue of not having acquired a proper first language in their formative years. Language skills really take root in the formative years, between ages one and five, and, if these skills are not acquired in this time, people, whether hearing or deaf, tend to always lag behind in language development.
Often, in the case of deaf children with hearing parents, there is, for various reasons, a failure to impart a strong base in a first language; the children may have rely on outside instructors or attend sign language classes for learning ASL. This is not the case in deaf families where the children pick up ASL skills as naturally as speaking children in speaking families pick up a speaking language.