TrueNAS not booting at first try, only when I shut it down and restart again manually

I have set up a small NAS from an old pc and it works great, but I noticed that when I need to shut it down, it won't boot up at the first try.

When I turn on the NAS, all lights goes on but my share is offline and the monitor shows "no signal".
If I shut it down manually by pressing the on/off button and then press it again to turn it on, it goes back working again... Why?