After installation the app needs to sync your current work item comments to capture the initial state. This might take a while depending on how many work item comments there are.
You can always retrigger this process manually later, but there is usually no need to.
The progress of a sync process currently running is shown directly on a work item panel or in the configuration section.
Limitations
Work items with over 1,000 comments cannot be synchronized during the initial sync. As a result, the timeline of a comment may not be accurately reflected.