I don't think those did AZW3.
You might check the device configuration setting. Right click the device icon when the device has been detected, configure.
See what formats have been ticked.
Calibre should have offered to convert the Library current format to one of the enabled formats.
Anothe place to check is Preferences: Behavior

referred Output format (I think this is just used when you do a manual Convert)
Another possibility, is there is a now an unreported code bug (I am not sure how many of those antiques

are being used these days. )
And there is a possibility that your old friend is dying.