Woodworking planer troubleshooting?

Hi, I've recently taken up wood working and am using a craftsman 5 1/2" bench planer to try and get some 8/4 lumber down to 1 3/4" in thickness. After running some of my workpieces through the machine, I noticed that the workpiece has a tzoidal edge where one side is not being planed as much as the other. Any help? It's the first time I've used the planer/joiner and I'm not sure what to do.