Most advice on nurturing advocates can be summarized in four steps:
1) Find people passionate about your brand (usually those that contribute the most)
2) Give them exclusive access, information, and possibly freebies.
3) Connect them with one another to build a community to share advice/feel a sense of connection.
4) Give recognition, training, and encouragement to do their work.
If this worked, we would see many more successful advocacy campaigns.
We don't because this advice works best when the community manager is tweaking the activity of existing highly active members rather than creating advice/expertise from scratch. It's much easier to change the behavior of current members than convert newcomers into advocates.
Second, the minutia matters a lot. The exact words you use in the approach, your own personality/nature on calls, the subtly appeals to increased social status will largely determine whether this succeeds. If you don't have world-class social skills, you're going to struggle.
p.s. One common problem is the brand tells the advocate what they want her to do. Far better to ask the advocate what she wants to do and provide all the resources/support to achieve that. Encourage accountable autonomy.