Several reasons could cause a furnace not to kick in. Possible issues include a malfunctioning thermostat, a tripped circuit breaker, a faulty pilot light or ignition control, clogged filters, or a malfunctioning blower motor. Insufficient fuel supply, a faulty thermocouple, or a problem with the limit switch can also be contributing factors. Troubleshooting these components or consulting with a professional can help identify and address the specific issue preventing your furnace from starting.