The only time I came across this before,a friend was
learning the ukulele;like me,he had previously been a
guitarist.The point is,he was gripping the chords/notes
way too hard.On some guitars this is the right way to
sound the notes properly;on ukulele,it is generally too
hard a grip.Lighten up a bit and see what happens,still
make the notes clear,not muffled or buzzing,but you do
not need to hold them down that tightly with nylon
strings.I found that I could grip too hard when I played
a soprano size,but with a bit of practice,it soon came!