The single most important factor that determines a successful Lead Generation program is Targeting — well above Creative, Copy or the Offer.
Focus Marketing programs on Suspects that are likely potential Customers.
Every Response to a Marketing program first needs to scrub out obvious throwaways to become a Lead.
Then, an assessment is done for Hot (I'm Ready to Talk to Sales Now), Warm (Not Now, Maybe Later), or Cold (Don't-Won't-Can't).
Most Leads will be Warm requiring an Engagement Plan to build a Relationship by Informing, Educating and establishing Trust until it becomes an Opportunity ready to hand-off to Sales.
This is often a weak area of many Marketing strategies.