Backstopjs site loading issue not solved by delay: what is wrong?

When I try to run backstopjs on certain sites (they all must have some dynamic rendering thing in common, though I don't know what), the screenshots generated by backstopjs only include the first piece of content, centered in the screen. Here's the URL to a screenshot: https://user-images.githubusercontent.com/41495147/63806833-1612b680-c8e2-11e9-9932-680864b470b7.png

I've already tried setting the delay to 5 seconds. I've tried waiting until the footer class is available before screenshot. No dice. What is going on? Here's my config file:

  "id": "backstop_default",
  "viewports": [
    {
      "label": "phone",
      "width": 320,
      "height": 480
    },
    {
      "label": "tablet",
      "width": 1024,
      "height": 768
    },
    {
      "label": "desktop",
      "width": 1280,
      "height": 1024
    }
  ],
  "onBeforeScript": "puppet/onBefore.js",
  "onReadyScript": "puppet/onReady.js",
  "scenarios": [
    {
      "label": "VMLYR Home",
      "cookiePath": "backstop_data/engine_scripts/cookies.json",
      "url": "https://www.vmlyr.com",
      "referenceUrl": "",
      "readyEvent": "",
      "readySelector": ".region-footer",
      "delay": 5000,
      "hideSelectors": [],
      "removeSelectors": [],
      "hoverSelector": "",
      "clickSelector": "",
      "postInteractionWait": 0,
      "selectors": [],
      "selectorExpansion": true,
      "expect": 0,
      "misMatchThreshold" : 0.1,
      "requireSameDimensions": true
    }
  ],
  "paths": {
    "bitmaps_reference": "backstop_data/bitmaps_reference",
    "bitmaps_test": "backstop_data/bitmaps_test",
    "engine_scripts": "backstop_data/engine_scripts",
    "html_report": "backstop_data/html_report",
    "ci_report": "backstop_data/ci_report"
  },
  "report": ["browser"],
  "engine": "puppeteer",
  "engineOptions": {
    "args": ["--no-sandbox"]
  },
  "asyncCaptureLimit": 5,
  "asyncCompareLimit": 50,
  "debug": false,
  "debugWindow": false
}```