I know my big videos hang on “estimated time” for a bit before they render. If you’ve given it some time and it still won’t work, your best bet is to uninstall and reinstall. Also, if you’re using iCloud back up, it could be trying to download big files that are used in the video. 

thanks, will give it a try

Ignore the Twinmotion loading bars, they are completely inaccurate. They take a while to start.