Are you sure all Light images have been checked? Are they the same size, type etc... ? What version of DSS are you using?
Are there enough Stars in the Images to register, you may need to adjust the slider to detect stars.. After clicking on Register Checked Images, up the top of the window you will see two tabs, one says Actions, the other Advanced. Click on the Advanced Tab and you will see a slider there, if you move the slider to the left, you will detect more stars, if you go right, less... you can check the # of stars being registered by clicking the "Compute the number of detected stars" button. I usually try to aim for over 100 stars, the more the better, but if you go too high it will take a long time to process.
