The answer
GFCIs trip within 25ms upon sensing a 4-6 mA current leak. If it won't reset:
- Upstream main breaker is tripped (no power)
- Ground fault or moisture remains active
- Line and Load wires were reversed during installation
- UL 943 self-test lockout reached (internal sensing circuitry degraded, unit permanently denied power).
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.