Devices were "managed" at one time, and now why are they showing as "unmanaged"?
Devices will revert back to unmanaged if they have not reported inventory showing "ManageSoft for managed devices" is installed in the past 30 days. The idea is that they are not reporting correctly, so should probably be readopted.
To adjust the period you can run
INSERT INTO DatabaseConfiguration (Property, Value)
VALUES ('IsManagedObsoletePeriod', 30)
Where 30 is the default.
If the property already exists you can update it with
SET Value = 30
WHERE Property = 'IsManagedObsoletePeriod'