If you are on a different board than Jetson devkit, it may be different.
On the Jetson devkit board, it is normal that the fan doesn’t start when the Jeston TX boots.
This doesn’t produce so much heat and normal convection is enough, no need to waste power for activating a useless fan.
When you run intensive tasks on CPUs and GPU, if it gets too hot, then fan should start. When it is cold enough later, it would stop.
When running jetson_clocks.sh, it forces the fan to run at full speed, because it is likely to be hot soon.
If you don’t care about power saving and just want to have the fan always on, you may start it in startup script such as rc.local. This requires root priviledge:
echo 255 > /sys/kernel/debug/tegra_fan/target_pwm