If you are missing lazily loaded images, try increasing setViewportHeight to a larger value, e.g. 16000.
If you are missing some resources, create a debug log using setDebugLog and parse it. The debug log shows a lot of detailed information about the conversion. It is useful for identifying parts to speed up performance.
If some JavaScript-generated content is missing, we recommend adding some HTML element with an ID (e.g. conversion-trigger) to the document when all the content is ready. Use the element ID in the setWaitForElement method. For example, your Javascript
// some content is generated by your JS
...
// everything is ready
let trigger = document.createElement('div');
trigger.setAttribute('id', 'conversion-trigger');
document.body.append(trigger);
and your API code
client.setWaitForElement('#conversion-trigger')