Prospects will be missing from the Prospect Assignments list if they have a historical plan only and the list is set not to include historical plans.

1. From Fundraising > Open the Prospect Manager's record
2. Select the Fundraiser tab
3. In the section for Prospect assignments click the Filters button
4. Mark the box to include historical plans
The list will now show all prospects for which this manager is current assigned regardless of the status of their plan.

Prospects with no plans who are assigned to the manager will always appear on the list.
The filter for Including historical plans is "sticky" so that the selected filter will stay checked when you revisit this tab later.