Transloadit task never finishes

I'm working on a mobile app that stores images on S3, merges them into a video and pushes them to YouTube. I'm currently looking into moving the process to Transloadit as it would solve a lot of issues I'm having with it, but I can't seem to get it working properly.

The flow I plan is:

1. Fetch images from S3 bucket
2. Merge them into a video
3. Push video to YouTube

I've put together the following template for testing purposes:

{
  "steps": {
    "get_images": {
      "robot": "/s3/import",
      "key": "****",
      "secret": "****",
      "bucket": "image-test",
      "bucket-region": "eu-west-1",
      "path": [
        "uploaded_files/a9f80e0c-48a7-4411-bab6-c218efccec6e.jpg",
        "uploaded_files/e056a30b-03fb-4e7f-8e91-edf8a66494c5.jpg",
        "uploaded_files/bce0be54-0fd9-4367-ae87-b670214a4830.jpg",
        "uploaded_files/652dbaf0-7b4d-4dbc-9b51-a07e79d116a9.jpg"
      ]
    },
    "create_video": {
      "robot": "/video/merge",
      "preset": "flash",
      "width": 854,
      "height": 480,
      "resize_strategy": "fit",
      "framerate": "1/5",
      "use": {
        "steps": [
          {
            "name": "get_images",
            "as": "image"
          }
        ]
      }
    },
    "youtube_upload": {
      "robot": "/youtube/store",
      "use": "create_video",
      "username": "Youtube User",
      "password": "****",
      "visibility": "unlisted",
      "category": "Film & Animation",
      "keywords": "blah",
      "title": "Test",
      "description": "Test"
    }
  }
}

The task never finishes execution. By deleting the steps, I've been able to establish that the first step runs and picks up the correct files, but the second step is never completed, suggesting that's where the problem lies. I've also observed that the demo for generating a video doesn't appear to work either, which is a tad worrying.

Any idea what the problem is?

Was able to resolve this issue in the end by adding "ffmpeg_stack": "v2.2.3", to the create_video step.