Hmmmm... You should have been able to get a nice even gradation by moving it up and down during the burn. It's best to make several up/down passes if possible, in the time available. Holding the card higher gives a softer edge. I had wondered if you were using your hands, as many printers do. It's something that's convenient because you always know where your hands are, but it does need practice.
Many printers like to burn in all the edges very, very slightly. It's said to "contain" the image but it shouldn't be noticeable. If it was obvious, we'd call it vignetting and have to do it differently.