First you need to check on the compatability. If you're running Vista you might run into some trouble
with alchohol if that's what you're using. DaemonTools light has been working great for me in conjunction
with anyDVD decrypor. I found that for some of the games I had to run the decryptor at the same time or
it wouldn't recognize the image.
Also, if you're using a shortcut and installed it with the cd and the game requires the cd during play, you
might want to check the file pathway and make sure that the program is running off of the image drive, not
the regular one it installed from. That's a "doh" moment, but plausible.
What game is it? That might help in some diagnosis. Some of the discs require special cracks installed
before it will let you run it, even with an image mounted.