I have three fleets of warships that are showing no availability when sitting in port.  I can still send them to a new destination when I manage them separately, but cannot plunder with them or send them to a new destination directly from the HQ screen.   Any idea what might be causing the glitch?

are you in a diffrent port then the fleets?

You mean that if you click on "plunder" you get the message "You cannot start any plundering without at least one fleet in port!"?
To plunder, the fleet have to be in the the same port where the screen is (the port name in the top bar, near Inbox, Fame, Gold, etc.)