ArcScene disappearing DEM's!

Hey All,

So I want to be able to view my DEM that I am using in ArcGIS10 in 3D. However, the file is very large and when I add it to ArcScene it computes it so that it only had two values under symbology.

However, I have smaller grids of the DEM that I have tried to import into Arcscene (will merge them later) however, I can't see them? Anybody know why this might be?

Thanks for any help in advance :D
Katie