Hi Clare,
Are you using featured image slider? Go to dashboard-> appearance-> customize->featured post/page slider-> slider options. Check the no of slides (by default its 4).
If the default no of slides is set to 4 then you need to add all 4 images and click on save and publish.
If you set to 2 then you need to add both 2 images. Add all the fields and your slider will work. Just try this once and if you have any issues then let us know again.
Thank you!