Usually one of five: slow load, weak hero, too many fields, no proof, or nav links leaking traffic. Fix in that order.
Diagnostic sequence
1. Load speed (LCP under 2s) 2. Hero clarity (outcome + credential + CTA in 4 seconds) 3. Form length (4-6 fields for top of funnel) 4. Proof (named testimonials, FCA number, reviews) 5. Nav links (strip, or keep minimal)
Target
6-14% form-conversion for paid traffic; under 3% usually means fundamentals, not design.