Why aren't my custom blocks auto-populating in Dawn theme?

when i add an image with text section for example, i get default blocks already populated. Im trying to get this to happen with my own sections.

Ive got presets working in that they can be added using the editor. I then tried using defaults to make blocks appear automatically but that didn’t work and it’s certainly not how the Dawn them does it. I cant see what im doing different to the image with text section in Dawn.