Is the preview black or just the final scan ?
If the final scan, have a look at settings -- contrast may be too high.
If preview, check that the slide holder is in the right place -- this may not be an issue if the film adapter is built-in but it certainly is on models with a movable mask.
Lastly, is the issue with slides in white frames because that can confuse the sensor? Solution is to dismantle the slide and put the bare film in.