I just moved to a new computer and copied over my Trackmania 2020 folder from the old computer. I see all my custom tracks, but for some reason none of the custom blocks or items show up. Mind you, I haven't used the editor in about a year. Did something change?

Solved it. Strangely Trackmania had created a new folder in the Documents folder, simply called Trackmania, rather than Trackmania2020. It had copied over my custom tracks for some reason, but not the blocks or items.