this can have several reasons. Please give us information on the following questions.
- No auto-download DEM is available in your area (where is it located?)
- SNAP cannot access the DEM directory (some things changed recently, are you sure you have installed all updates?)
- No topographic patterns are reflected in your intensity (do you have topography within your area?)
- The spatial resolution of the downloaded SRTM doesn’t reflect the variations in the image (which one did you select and what’s the spatial resolution of yur CosmoSkyMed data?)