What makes you think that Robin, who had her DF since age 8 and Fujitora, an another admiral are not awaken?
Never thought on Robin, this is debatable.
The usual paramecia awakening was defined as being able to
transform the surroundings into the element, e.g. transforming buildings or the ground into strings. Ryokugyu merely made plants grow out of the ground and walls, which is what plants naturally do. He didn't transform the ground and building into wood. That's different. Same for Robin. Also, not all paramecia, let alone DFs in general have the same awakening. He are not even sure yet whether Greenbull is a paramecia, a logia (since he turned into wood himself) or even some kind of mythical Hito Hito no Mi.