It may be caused by stretching 1 duct to create the next

try double clicking on the duct to open up the edit part take-off page, and then close it down and see if that fixes it

does it still show as o.oo area?

its a common issue when creating straight ducts by stretching them and it wont show any area until you open the part up or change the specifications.

there is a post previously regarding this which explains it better than me

hope that fixes you issue

Thx