Hi KarlKarpuk,
In most cases this happens if you build a model with the Model Builder, but the Bing Maps service has limited available imagery for your area of interest.
To test this out, I just created a model of Veisjärv, Estonia (Estonia) using the Model Builder (September 2015 release of InfraWorks 360), and was able to display the aerial imagery for the terrain only after reducing the Bing Maps imagery tile level to level 13 or below.
To do so, right click on your ground imagery in the Data Sources panel, and choose Configure. Click on the raster tab, then modify the tile level resolution and click Close and Refresh.
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Principal Specialist, Infrastructure