Not all glue sticks are created equal  

Not all glue sticks are created equal

With my previous printer, I had been printing PETT, nylon, and other materials on glass using Scotch glue stick. When I got the Prusa MK3 kit, it came with a tube of "Kores" glue stick, but I kept on using my Scotch glue stick to do PETG prints. (Note: I tried bare PEI, but the PETG stuck much too well to the bed. I tried Windex [actual brand] on the bed, but the prints were still stuck too well.) However, after several years of use, the Scotch glue stick finally ran out. Then I tried using the Kores for printing with PETG. It was a disaster. No matter how much I played with the Live Z setting, I could not find a setting where the first layer would actually stick to the bed satisfactorily.

So I cleaned off the bed with water & dishwashing detergent, then IPA, and then applied a new coating of Scotch glue stick. First layer adhesion is now perfect again.
To be clear, Scotch makes more than one kind of glue stick. The specific one that I'm happy with is "Scotch Craft Stick" with a part number 003-ESF in the label.