This Facebook ads setting is limiting your leads.
When you create a Facebook lead form, do not forget to go to the Settings tab. There's an option there that most advertisers don't even know exists.
It's "Sharing," and by default it's set to Restricted. This means that "by default, only people who are delivered your ad directly will be able to see and submit this form."
There's no other example of when this happens. If you run an ad to promote the purchase of your product, you don't restrict the ad so that someone who shares it with a friend can't buy the product. That would be crazy.
While there may be some reasons to restrict leads to people you paid to reach, it's got to be the exception rather than the rule. Yet, the majority of advertisers don't realize they're doing this.
If you ever publish your lead form to the news feeds of your followers, you'll want all of your followers to be able to complete it.
If you have lots of budget behind a lead ad or get lots of engagement, you'll want people you didn't pay to reach the ability to complete the form. Wouldn't you?
It's unlikely you'll have lots of organic leads, but you will have some. Only if you allow for it, though.
Once you set your form to Open, Ads Manager still won't report on organic leads. You can see these within your export or by using the Organic Source filter in your Leads Center.
I find it weird that this is the default. What do you think?
You can read my blog post on this, too:
https://www.jonloomer.com/are-you-restricting-your-meta-ads-lead-forms/
