It used to be, with older versions of Captivate, that a imsmanifest file wouldn't be published if there were no scored interactions included in the piece.
I think that was fixed around version 5...
BUT may be worth a check. Any scored interactions (i.e. Quiz Slides or otherwise custom tracked/scored interactivity) in your piece?
If not, include one (i.e. 'Click Here to Begin' button on the first slide, and score that button), publish, and see if that works?
Alternatively, are you publishing to a .zip file? If not, try that and see if a manifest is included...