What does Wittgenstein view as a problem with learning a language using "ostensive defining"?

Wittgenstein clarifies the problem of communicating using a human language when he discusses learning a language by "ostensive defining." For example, if one wanted to teach someone that a pencil was called a "pencil" and pointed to a pencil and said, "pencil," how does the listener know that what one is trying to convey is that the thing in front of me (e.g., the entire pencil) is called a "pencil"? Isn't it possible that the listener would associate "pencil" with "wood"? Maybe the listener would associate the word "pencil" with "round" instead (as pencils are, usually, in fact, round!). Wittgenstein writes regarding several possible "interpretations" which may arise after such a lesson. The student may interpret your pointing at a pencil and saying "pencil" to mean the following: (1) This is a pencil; (2) This is round; (3) This is wood; (4) This is one; (5) This is hard, etc., etc.

As example of "Ostensive defining" is pointing to an object and saying it's name. Wittgenstein sees "ostensive defining" as a problem of communication because simply linking a word to an object carries with it ambiguity. For instance, if someone pointed to a pencil and said, "pencil", the listener may not know exactly what the word refers to. It may refer to the pencil itself, the shape of the pencil, the material of the pencil, the quantity of the pencil, or any other quality of the pencil.