If you got as far as a reboot after flashing the factory image, then you probably just didn't wait long enough or your download was corrupt. Try downloading the image files again and reflash. I don't know about hours, but first boot does take a good long time.
The OTA update is built for 4.4.4 -> 5.0 so you would have to do all the updates in between. Or you could flash the factory image and go straight to Lollipop, but you would lose your data. Which route you take is up to you.
Thanks! I´ll download the images again and reflash! Hope to get any luck this time!