GitHub Actions workflow not triggering -- common reasons

GitHub Actions workflow does not run on push, PR, or schedule. The workflow file exists but no runs appear in the Actions tab.

Check: (1) File must be in .github/workflows/ on the DEFAULT branch for schedule triggers. (2) YAML syntax error: validate with actionlint or yamllint. (3) Branch filter: on.push.branches must match the pushed branch. (4) Path filter: on.push.paths might exclude your changes. (5) Workflow disabled: check Actions tab for disabled workflows. (6) Fork: workflows do not run on fork PRs by default (security). (7) Commit message: [skip ci] or [no ci] disables workflows. (8) Permissions: repo must have Actions enabled.

GitHub Actions has many trigger conditions. A workflow only runs when ALL conditions in the 'on' filter match. Mismatched branch names, path filters, or disabled workflows are the most common causes.