A smart campaign is configured to clear the field value for a field, however, when the smart campaign ran, it didn't appear to have cleared the field values and the campaign results display no activities.
Check to see if people qualified for the smart campaign and became campaign members by clicking on the 'View Campaign Members' button. If there were people that are in the campaign members list, then it means people qualified and the flow step did not process.
If people did qualify, then check [Admin > Field Management] to see if there is a block field updates enabled on "Flow change data value" - In this case, temporarily disable the block field updates on "Flow change data value" for the field, run the campaign so that it can clear the values on the field and then re-enable the block field updates.
The smart campaign did not return any activities in the results if the flow step was only configured to change the data value of a field that has block field updates enabled for "Flow change data value"