Buzzing can be caused by several things. If you don't hold a string down cleanly, just behind the fret, that can do it. If a finger is too close to a vibrating string, that can, too. Often it is caused by vibrating string touching a fret (sometimes a string too low, too close to the frets - sometimes a fret sticking up a bit). Problems with the nut or the saddle can cause buzzing. If your uke is amplified, it may have wires inside vibrating. Sometimes simply changing the strings will get rid of a buzz. That's not a complete list - there are others. As you will know, a uke (like most musical instruments) works by using controlled vibrations. When unwanted vibrations creep in, it can be difficult to diagnose.
I'd advise you to start by checking that your chord fingering technique is not doing it. Then, it it's not you - it's the uke.
Best of luck,
John Colter.
