I noticed that for a media embed block with a caption, changing the alignment of the block does not move the image unless the alignment of the text (caption) is changed to correspond with it. Eg in the block layout configuration, if the block alignment is set to right, the image shows up in the same place on the page (default, or left) unless the text alignment is also changed to center or right. I’m not sure if this is the intended behavior, but maybe there is a reason for it. At any rate, I spent a bit too much time troubleshooting only to realize it was the caption alignment that was causing me trouble. Maybe someone else will see this and save some time.
I am using Omeka S 4.1.1 and the theme is Lively, if that matters.