Brian,
We've had this issue also. As stated in several other posts, insure your users are in compatible IE versions. We found on certain PC's (mostly non CAD) that we had to alter a couple of settings in CV Options. Go to File tab, Options; at the top change pulldown to Defaults and select Drawing Viewer. There check the box for Use background color, suggest leaving the background white. You may also want to select the Monochrome display. This way your drawings open as black lines on a white background, otherwise the print may be another color (ours was blue and harder to read).
Lastly, before updating to WF10, I had times when a drawing would seem to open but wouldn't show. Clicking in the empty white space would cause the file to appear. Not sure the cause, just know it worked.
Good luck.
Mark