Short answer
HubSpot's KB: sessions only appear if the tracking URL was viewed during the report's timeframe, and the destination page must have the HubSpot tracking code. If sessions land in an unexpected bucket, review how HubSpot categorizes sources.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Test the URL yourself in a private window.
3. Related
Tracking URL builder and tracking code check.