Your theme normally dictates how the contents are shown in your pages.
Sometimes, blocks may not show up in full-width properly. When this happens, most of the time things get fixed by using a full-width page template.
Themes usually provide this type of template. To see if this is the case, please see if your page attributes have something like this:
If your theme doesn’t have a full-width page template, there are a few plugins that that provide this type of functionality.
Here’s a free one from the WordPress Plugin Directory called Fullwidth Templates for Any Theme & Page Builder (we are not affiliated with the plugin)
Your mileage may vary depending on your theme though, so if you find that this method doesn’t work, you can also manually create your own Full Page Template. ThemeIsle has a great article discussing how to do this.