Sometimes you'll use up the 5MB allocated for local storage on the YouTube domain. (likely other chrome extensions that abuse local storage we will only use a maximum of 5KB).
You can clear the local storage for youtube.com by:
Type "youtube.com" in the search box
Click the "www.youtube.com" entry
Click the "Local storage" tag
Click the "Remove" button
Refresh https://www.youtube.com and log back into the vidIQ extension