[CRJ Autopilot not engaging]

So if I dont use the EFB before I leave the gate to set any state, then the engines are on and I taxi out the AP works perfectly. I am also using NAVIGRAPH to import my flight plan so I hit NAV around 500 feet then AP and away I go. BUT -- if I set TURNAROUND so the engines are off and then proceed with pushback, when I then set TAXI state later - or even if I dont set any state and simply start engines, load my flightplan and taxi and takeoff, I can turn NAV mode on, but when I hit AP button it refuses to engage. I have tried YAW dampers setting it makes no difference. I'm missing a step somewhere. I did turn APU off but there must be something else and I dont really want to watch an hour of video if I can avoid it?