This issue could be due to a simple checkbox being unchecked in content/page background settings. Try this:
1. Select Content
2. Select the Page/Content Section you have your issue on
3. Select Design this page in the sidebar
4. On the right side of the design panel, you'll see Sitewide, select Background at the bottom of the panel
5. Scroll to the very bottom. Check off the box that reads "Resize background image based on device type"
6. Select Save
7. Select Publish