hi mike,
There might be two reasons for this issue
1. Structure of your page has changed since the activity was last saved.
The missing selectors might be reached using Browse mode. It is recommended that you delete and then re-create each experience to ensure that your content looks as you expect, as indicated in the warning message.
2. The element on which you have done modifications is loading after modifications getting fired so it is unable to find the element to perform modifications and throwing error "missing or invalid element selector".
This might help you to understand more about Element Selectors Used in the Visual Experience Composer
Thanks,
Gauresh.