Could be a leak at the upper end.
Also, because the embouchure on soprano is much firmer than on alto, especially at the middle and high end, as was suggested you might not be loosening your jaw enough on the low notes. Try relaxing a bit at the low end, and if that doesn't help, have the horn checked for leaks.
Also, because the embouchure on soprano is much firmer than on alto, especially at the middle and high end, as was suggested you might not be loosening your jaw enough on the low notes. Try relaxing a bit at the low end, and if that doesn't help, have the horn checked for leaks.