Not leaving on good terms would be the reason. My employer is unpopular in this regard. If an employee quits, and then needs a document such as this, the company has usually refused to provide it.
But all these responses help. I could try to get the letter before quitting, hoping by the time I apply for PR, CIC does not consider the letter to be an old one and disregard it. Thats the best I can hope for.
Thanks folks!