you might want to take all the screws out of the top section, assemble it on the stand and then screw everything back together. most of the time when working with lumber from home depot or lowes , all of the boards have a slight warp to them, and when you assemble the frame,the whole thing will be off in one corner by 1/2 an inch or so. it also helps to check your miter saw to make sure you blade if perfectly perpendicular to the cutting surface, even if it's off by 1 or 2 degrees, the whole stand will be out of whack.