Generally, this happens when a location/location group attached to the campaign was updated a few minutes before the campaign scheduled time. Due to this, the data is not synced with snowflake and the campaign process with 0 guests.
As of now, the campaigns are running through Snowflakes and it takes up to 1 hour to sync the DB with snowflakes. It is recommended that when a location/location group is attached to the campaign, it should not be edited in the 1 hour duration before the campaign scheduled time.