I have a new Creality Ender 3.
I suspect that I have not adjusted the eccentric nuts correctly, on the X-axis head carriage mounts.
Even after a glass bed upgrade, using the Level Corners routine of the TH3D firmware, I can get the head to scrape a sheet of paper all 4 corners but that same sheet of paper does not then scrape the head in the center, unless I fold that paper in half.
I could understand this if the rail on which the hot end travels is very slightly higher at the side opposite the extruder. I have tried turning the eccentric nut on that side until the head does trap the paper, but when I then repeat the Level Corners routine, the gap at the center has come right back.
I already adjusted the eccentric nut on the extruder end because the wheels on the hot end carriage were showing an accumulation of brown dust in a ring around each wheel, which I heard was likely a sign that the carriage was "too tight." So basically, I am messing around with the eccentric nuts at both ends of the X-axis rail, chasing two problems at the same time, but I don't really know what I am doing.