Publish issues. Action script 3.0 doc > SWF okay but MP4 not working

I'm relatively new to Animate but I've been using Adobe products for 25+ years and I've done numerous simple animated gifs before. I've created an (approx) 45 second animation which tests fine as an SWF file but when I try to publish as mp4 gets stuck/does not finish export/render. I've googled to no avail. Not sure if it's my export settings or ?????? HELP!