Leaders lack timely, accurate and compelling information in a usable form. While it is true leaders seldom have or take the time to read research reports and other lengthy documents, when they do want information on a particular issue, they usually want it immediately and they want it to be accurate, understandable and compelling. Particularly in part-time legislatures where professional staffing is limited. Most people who call themselves 'advocates' provide a little information too late and then complain. That doesn't work.
Leaders perceive advocates to be partisan.
The perception of nearly all the leaders interviewed is that most advocates are liberal Democrats. In state after state. I'm not compassionate, and so on. That's just human nature. I believe the advocate tend to deal with Democrat members more than they deal with Republican members.