Ads keep switching to the default

I have a series of 4 ads in rotation as well as a default ad set for the ad place. Frequently, the 4 ads stop showing and the default ad shows instead. The ads have not expired and all show as active.

This usually is corrected simply by logging in, saving an ad without changes and refreshing. HOwever, my client is quite annoyed that this continues to happen. I have since removed the default/fall-back ad and am hoping this helps but any insight would be greatly appreciated.

https://www.ads-software.com/plugins/simple-ads-manager/