Facebook advertising is hard for health-related issues - people aren’t as willing to like, comment or click because they sometimes don’t want their friends and family to know about these issues (Facebook and other related social media is notorious for users only wanting to represent their “best selves” to the world). That could be one thing coming into play here.
Consider “solution-based” ad platforms, like Google AdWords or Youtube. People use these platforms to find solutions to problems or direct answers to questions. I’m sure you could find some cool way to promote your community there, if you havent looked into this already.
For Facebook, if you have the budget, try this:
Create a piece of content like a video or a blog post that is super valuable to your target audience. Maybe take one of your most discussed topics in your forum and turn it into content (like “Top 5 Healthy Snacks For Type 2 Diabetes” or something like that) and boost it to that desired target audience. Put a Facebook tracking pixel on the blog post. This lets you create a custom audience that you know are specifically interested in Type 2 Daibetes recipes, because they clicked on your post.
Run a retargeting ad to that custom audience that promotes your community’s discussion of the recipes. You are now sending an ad directly to people who have expressed interest in your topic and they have seen your brand before (your blog). Because they are more familiar with your brand and what you have to say, they will theorhetically be much more likely to click through on your offer.
Alternatively, take a look at your current boosted post. Are people clicking through and then dropping off? It might be your page. If they arent taken directly to the content you’re mentioning and able to easily contribute, you wont see much from your ad. Work on optimizing what that experience is like, because a confused mind will never take action.
I hope this helps!!