Hello EV_91,
Can you please check, if the bounding box (Region) does include all the area of interest? If not, it should be manually adjusted prior to the dense cloud generation.
Also missing dense cloud areas could be caused by the lack of the image overlap (low number of matching points for the image pairs), but such areas can be interpolated - if you use Extrapolated option in the Build DEM dialog, the surface would be extrapolated up to the bounding box sides, therefore you will be able to generate the orthomosaic for the whole area.