I mapped the build plate temperature on a 1" grid on my Creality Ender, and found it is both less than sepecified by about 5°C, and uneven -- varying by about the same 5°C amount. The thought then occurred to me that the thermal coupling between the alum baseplate, and the glass build plate, might be less than perfect. The, admittedly thin, layer of trapped air, and the presumed imperfections in the surfaces, may be partially to cause of the aforementioned issues.
Sooo, I wonder if a layer of heat sink compound might help? It should help, we'll see. Has anyone tried this, and did it work, or just make a mess (or both?)