Try pulling the entire z assembly off by un-threading it. lay it on a flat surface to make sure the z rails aren’t twisted. The z motor mount or the tool holder can twist the rails while tightening it down.
If you assembled the middle assembly and then put it on the rails take out the 4 3″ bolts and put them back all while on the gantry. Some people have assembled it and tightened it down before putting it on the rails and it tends to not tighten up at 90 degrees.