Some parts of a model are not displayed in Revizto.
To optimize model rendering, Revizto draws only one side of each polygon. You do not see the polygons because their surfaces that face your point of view are not displayed.
Go to Preferences > Graphics > Rendering and select the Double-sided materials checkbox.
This will display both sides of each polygon.
The figures show a model with the Double-sided materials checkbox cleared (left) and selected (right).