You pushed a commit, opened the Actions tab, and there's nothing there. No red X, no yellow dot — the workflow simply didn't run. There's no error to read, no log to grep, because a workflow that doesn't trigger produces no run at all. The decision happened before any runner was assigned, inside GitHub's event-filtering logic, and that logic is more surprising than the docs make it look.
Almost every "why did my GitHub Actions workflow not trigger" report comes down to one of a handful of causes: the workflow file isn't on the branch you pushed to, your branches filter doesn't match the ref, or — the big one — you combined branches and paths without realizing they're ANDed together. Here's each cause with the deciding rule and the fix.
1. The workflow file isn't on the target branch
GitHub reads on: triggers from the version of the workflow file that exists on the branch receiving the event — not from your default branch. If you added .github/workflows/ci.yml on main but push to a feature/x branch that branched off before that file existed, there's no workflow to trigger there.
# on main, but feature/x branched before this file existed
