I still have Windows 7 on one of my computers. I also have the wallpaper set as a slideshow so it displays a different background every minute. I noticed yesterday that my backgrounds where not changing. I had no problems setting the backgrounds manually. You can usually force a background change by right-clciking on the desktop and clicking on Next desktop background in the resulting menu. It did not change.
I went online and found a couple solutions that didn't work. So, I right-clicked on the Windows 7 desktop and clicked on Personalize. I clicked on the Desktop Background icon and checked my settings and everything was the way it should be.
I decided to try a different theme to see what happened. I just clicked on the Architecture theme. When I clicked on the icon, it changed the background. Then I right-clicked on the desktop and then clicked on Next desktop background. It went to the next background.
So I went back to my pictures again and this time it worked. So just try changing your theme and then switching back to the one you want.
Other options.
I came across a couple of other options for fixing this issue. I list them here in case the above doesn't work for you.
The most popular one seems to be edit your power plan. Type Edit power plan in search and click on it when it appears in the menu. Then click on Change advanced power settings.
In the lsit the search for Desktop background settings. Click on the + sign before it to see the Slide show. Click on the Settings tab to change your option. Set it to Available. With a laptop, set both Battery and Plugged in modes to Available. Then OK.
Another option suggested is to empty the theme folder.
Navigate to C:\User\(your account)\AppData\Roaming\Microsoft\Windows\Themes
In this folder on Windows 7 you should see at least two files. One called slideshow.ini and the other TrascodeWallpaper.jpg.
Delete the TrascodeWallpaper.jpg and then open up slideshow.ini in notepad or similar program, select and delete all the text of the file and then save it. If you have problems doing it, right-click on the file first and go into properties and uncheck the Read-only Attributes.
On my dual monitor Windows 10 system ther are 4 files; slideshow.ini, Transcoded_000, Transcoded_001, and TranscodedWallpaper. These Transcode ones are just the pictures with no extension. So if you are having this problem on Windows 10 do the same. Erase contents of slideshow.ini and delete all three trascode files. Transcoded_000 and TranscodedWallpaper seem to both represent the picture that appears on the first monitor.