Some fraction of the full professors seem to lose momentum, often it turns out because they never had much to start with. But they are tenured, and we need to give them a chance to deliver on their promise, whether they wish to or not. They cannot be channeled into administration or extra teaching, until they have proven unable to maintain a research career. It is up to their colleagues to figure out ways to give them a chance--incorporate them into your research projects, mentor them, face them down with the facts of their current performance and with the possibility of getting what they need to do their work. It's best they not reach the Alcoholics Anonymous "rock bottom", but if that is necessary so be it. It's dreadful to be a full professor in a research university and not be research productive. Better to move to another institution, to take up an administrative position, to find another path, than to be perceived as "dead wood." By the way, that perception is not just a matter of not publishing or getting grants, it may be about the work you do that is not seen as deep and challenging.
Fashions in research change, so perhaps the work you do is not what others consider important. But there is a community out there that does appreciate your work, and it is crucial that your colleagues know that. I've seen too many senior faculty pooh-poohed, and then a few years later their work is back in fashion--for less senior faculty tend to be arrogant about what is the most important research--but they never see how their own work is being superseded by more junior faculty and those at other institutions.