High Sierra 10.13.4 Mail App Problem

Hello,

I am running High Sierra 10.13.4 on a MacBook Pro. Recently I have noticed that the mail app icon in the dock no longer shows the number of unread / new e-mail messages present in my Inbox. I have checked the Mail preferences 'General' settings, and the 'Dock unread count' setting is set to 'Inbox Only'. The 'Message notifications' setting is also set to 'Inbox only'.

I have tried rebuilding the Inbox, but it has not effect. How do I restore the dock mail icon to showing me the count of unread emails?