mp4/itunes issues

I have a MacBook Pro laptop and I have always transferred movie file (mp4) from my computer into iTunes via the add to library and since the new update it will not work even if I try dragging and drop nothing happens. I have check every possible folder and that includes the home video folder and nothing is there. I wanted to put this movie on my iPhone 8 to watch on a long car ride. I just 2 weeks ago was able to add other MP4 files to iTunes via the add to library and now it's just not working. Is there a new way to complete this with the new update or have they just blocked you from adding movies unless you buy them from Apple?

Thanks for any help

D