If you are speaking of the Washoe studies, those studies were inherently flawed. The research wa extremely sloppy, criteria varied from session to session, and the entire experimental design was such that the researchers could not help but see exactly what they wanted to see.
A) Any motion that even approximated a sign was counted as spontaneously communicating with sign.
B) The researchers and research assistants were not native, or even fluent signers themselves.
C) The primates involved never used any form of a sign spontaeously, nor did they attempt to teach the communication they were supposedly being "taught" to other primates or to their young. Only upon prompting, to include an assistant actually molding their hand into an approximation of the sign, did they actuallly produce the sign.
D) Motion that was recorded as being spontaneous communication with sign included those motions that could not readily be recognized as a formalized sign. In addition, they were used out of context, and no relation to the object for which the researcher was requesting the sign.
E) As the primates never used sign language for spontaneous communication, nor did they attempt to teach it to other primates, they did not acquire the language of signs.
Check out the book, The Other Side of Silence for a realistic review of the inadequacies of these types of studies. I also believe that Oliver Sacks book, I See a Voice mentions these research studies, and analyzes the results from a very scientific point of view.