Hello  @KateD 

To check if it is related to mobile device itself, please confirm other apps work fine on the same phone.

Did you try to uninstall the APP and install the latest kasa app in Goolge play or APP store,did you recently update kasa app / phone OS before this happens.

You could try a different phone as well. 

Please let us know the model of phone and its IOS or Android version, a video link showing the concern, and APP version , thank you.