Workflow sometimes doesn't trigger

I have a workflow set to run on a commit to a pull request. When it runs it works fine but sometimes it doesn’t run at all. Nothing in the actions tab and the pull request just sits there with

build Expected — Waiting for status to be reported

Here’s my workflow but given that it does work sometimes I don’t think it’s the issue.

name: Dart CI

on:
  pull_request:
    branches: [ master ]

jobs:
  build:

    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v2
    - uses: subosito/flutter-action@v1

    - name: Install dependencies
      run: flutter pub get
    - name: Generate models
      run: dart models/main.dart models/examples lib/models
    - run: flutter format lib/models
    - name: Format
      run: flutter format --set-exit-if-changed lib
    - name: Lint
      run: flutter analyze lib models

As a work-around you can close and re-open your PR.