Hi,
Sorry for not replying – the email notification on my site broke after an update and I didn’t catch your question.
It’s hard to tell what could be the issue – are you sure you are editing an item that has child items? – if so, try loading the form in the browser emulating a mobile device using the dev tools (in Google Chrome or the new Edge – hit F12 and click the icon in the top left corner to toggle mobile mode).
Any errors in the Console tab in the dev tools?
Alexander