A couple of questions
Do the slides show the same softness?
Are they well exposed?
If the projected slides look fine, then you need to work on scanning. Possibly the scanner is substandard, out of focus or your not doing the best job with the scanner.
If they are soft but well exposed, is the focus correct?
If the focus is correct, then use the camera on a colorful subject, with "sharp" objects to focus on. Shoot the camera at a range of f stops and see if the lens has a range where it and you are happy.
If the exposure is off, all bets are off. Exposure and focus are "absolutely essential" to getting a good scan. Correcting poor exposure significantly in a scan can be done, but yields crappy results. Focus can be helped minimally with sharpening, but then crappy input yields crappy output.