Low activity is a relatively simple problems to diagnose.
If you have low levels of activity, you have three questions to answer:
1) Does the target audience know the community exists?
Let's call this awareness. Does the audience you're trying to reach know you exist? Have you had regular, repeated, exposure to this audience? What is the link of communication from what you have done to the target audience? How do you know they actually read the messages you sent out?
If the audience doesn't know you exist, they can't visit. If you want to resolve this, you need to engage in a lot of direct outreach, relationship-building, and promotional activities.
2) Does the target audience visit the community?
If the audience doesn't visit, they can't participate. This accounts for 90% of the communities struggling with low activity. The audience knows the community exists, but don't visit.
This is one of four problems.
1) they don't have a regular trigger to visit. Create a trigger.
2) they are not interested in the community concept. Refine the concept.
3) they don't want to. Build a relationship with that.
4) they don't have the ability to visit (lack of time/internet).
3) Does the target audience participate when they do visit?
If the audience visits, but doesn't participate. This is because they haven't found something within the community itself which motivates them to participate or they're not able to participate due to some technology problems.
There are two ways to resolve this.
The first is to tweak your menu of discussions. Make sure you have high-quality, topical, self-disclosure discussions which prompts members to participate.
The second is to nudge members to initiate their own discussions. This has to rely on either appeals to resolve a problem, get support, or build their status amongst the group.
If technology is the problem, you need to diagnose the entire newcomer to regular conversion funnel and resolve the areas where members drop-out.
Use data to answer these questions.
Once you know the problem you can create a solution. The solution is the easy bit. Most people simply don't identify the right problem. They try to change/tweak the platform without realizing most people don't visit the platform.
Identify the problem, test different solutions.