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

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.