Good advice here and I will also add my .02, fired cases are not as square as unfired brass so there going too be a little uneven because of the rim/extraction groove, I have turned the case after the trimmer has stopped cutting while its still running and get a more accurate cut.It does take some time. Maybe 20 seconds per case in my experience to get it really done right.
A couple of suggestions, make sure the screw that holds the clamp arm is reasonably tight so the case will move just a bit.
Make sure you lube the cutter shaft and the bearing surface between the block that controls the length and the adjustment cylinder and the frame. I watch the oil in the gap there and see it pool up to the same point and I get very consistent results. I would have to do a video to show it well though.
Play with it a bit more. I found once I got mine setup it was quite consistent and really easy to use.