Groups not working in extend

My groups are not working in this extend. What am I doing wrong?

fields:
  mainlayout:
    label: Grid
    title: Section layout
    type: layout
    layouts:
      - "1/1"
      - "1/2, 1/2"
      - "1/3, 1/3, 1/3"
      - "1/4, 1/4, 1/4, 1/4"
      - "4/12, 8/12"
      - "8/12, 4/12"
      - "7/12, 1/12, 4/12"
      - "4/12, 1/12, 7/12"
      - "4/12, 2/12, 2/12, 2/12, 2/12"
      - "2/12, 2/12, 2/12, 2/12, 4/12"
      - "6/12, 2/12, 2/12, 2/12"
      - "2/12, 2/12, 2/12, 6/12"
      - "6/12"
      - "5/12"

    fieldsets:
      blocks:
        extends: fieldsets/blocks

and the extend

label: Blocks
type: group
fieldsets:
  Layout:
    label: Layout
    type: group
    fieldsets:
      - nestedlayout
      - columnsetting

  Text:
    label: Text
    type: group
    fieldsets:
      - heading
      - tagline
      - text
      - list
      - quote

  SpecialText:
    label: Special text
    type: group
    fieldsets:
      - code
      - markdown

  Elements:
    label: Elements
    type: group
    fieldsets:
      - button
      - alert
      - notification
      - stat
      - table
      - accordion
      - line
      - spacer

  Media:
    label: Media
    type: group
    fieldsets:
      - image
      - video
      - gallery
      - logo
      - icon

  Components:
    label: Components
    type: group
    fieldsets:
      - breadcrumb
      - modal
      - banner
      - form
      - pricing
      - comparison
      - slider
      - slidertabs
      - slidercards
      - slidermarquee
      - testimonial
      - timeline
      - team

  Header:
    label: Header
    type: group
    fieldsets:
      - languageselector
      - logo
      - menu
      - search
      - expander

  Footer:
    label: Footer
    type: group
    fieldsets:
      - descriptiondetails
      - languageselector
      - heading
      - logo
      - menu
      - search
      - text