Could be:
-Uneven Frets- a high fret can cause a buzz- a proper setup could fix this
-Humidity issue- low humidity can warp your uke...
-Bad strings- Sometimes just changing your strings can really help.
-Operator Error- are you pushing too hard on the strings, or not hard enough? Are you pressing evenly in between the frets, and not on top of them? Practice practice practice is the only way to correct this.
I'm sure there are more things that can do that, its just all thats in my head at the moment...