There are many types of trust, but SMEs should be particularly attuned to two types that are universally important. First, people must be able to trust your skills or capabilities, and second, people must be able to trust your motives.
Trust starts with competence. If your audience does not have reason to believe you are capable of performing the desired tasks, then they will not trust you. When people trust your skills, they believe that you have the technical competence to perform a task and obtain a desired outcome. They believe you can accomplish something that they cannot do on their own.
When someone trusts your motives, they believe that you will care for their interests above your own. When someone fully trusts you as an expert, they will give you power, in the hope that you will accomplish a desired outcome, even if that outcome runs contrary to your own desired outcome.
Typically, we trust people who have excellent skills and who do not have ulterior motives. If people do not trust you as an expert, they will not follow you or give you power. It’s that simple. Either they do not trust that you can produce the desired outcome, or they do not believe that your advice is in their best interest. They either do not trust your skills or they do not trust your motive.