I've found misinterpretation and usability to be the most common causes. Eg. the capabilities are not clearly mentioned and so users sign up and see what can be

I've found misinterpretation and usability to be the most common causes. Eg. the capabilities are not clearly mentioned and so users sign up and see what can be done with it. And then decided that its not worth it. Or users sign up and the interface is not intuitive enough to guide them through the steps of what they need to do. Simplifying things helps a lot in these cases.