The fingernail is going to give a sharper sound but that's not really buzzing. If it's buzzing after the nail strike is over (i.e. during the first part of sustain) then check your technique and how hard you are strumming. You want to brush the strings parallel to the fretboard, if you are pushing downward (toward the top) with your fingernail as you strum then the strings vibrate more in the vertical plane than the horizontal (strings don't actually vibrate perfectly in any plane, actually, it's more of a rotary motion).
Sometimes, for a very percussive sound, you do want to strike the strings more or less vertically but in that case you're also usually palm muting so buzzing isn't an issue.
Finally, is the uke drying out? If you're not keeping it well humidified the first sign is usually that the top begins to shrink, lowering the action. If the uke is set up perfectly, as most KoAlohas probably are, there isn't much room for lowering the action.
John
