Lead Generation Campaigns are not just a fluke where you create ads and copies and hope for genuine sales enquiries.
Creating funnel approach can get you towards the goal of achieving genuine leads that converts into sales.
Understand your target audience
Create the target locations strategy
Use detailed targeting and custom audiences
Facebook's Blog on Tips for lead generation
https://www.facebook.com/business/news/tips-for-lead-generation-on-facebook
Hootsuite's guide or gist on lead generation through Facebook Ads
https://blog.hootsuite.com/how-to-use-facebook-lead-ads/