We’ve investigated and it’s proving to be very difficult to narrow what’s going on and why.
Here are a few follow up questions:
1. Have you edited the theme code in any way, like header.php?
2. Did you previously have other slider plugins installed?
3. Did you ever use a cache plugin at any point?
As you can see in the demo, the slider is still working fine:
The only other thing we could advise to try is to try deactivating all other plugins, but Nivo in case there’s a conflict there.