When a chord sounds bad, it is usually one string, and it takes under a minute to find.
The test
Hold the chord. Now play one string at a time, slowly, from the thickest down. Listen to each one until it stops ringing.
Each string does one of three things:
- It rings. Leave it alone.
- It buzzes. That finger is too far from the fret, or not pressing hard enough.
- It goes thunk. A neighbouring finger is lying on it.
Fixing the one bad string
For a buzz, slide that finger closer to the fret in front of it, the one nearer the body of the guitar, right up against the metal without sitting on top of it. You will need far less pressure there. Most people press harder when they should be moving closer.
For a thunk, look at the finger next door. It has probably flattened out and is touching the string below it. Curl it so the very tip comes down, like you are holding a tennis ball.
Try it now
Do the test on G, then on Em.
Fix one string at a time. When all of them ring, take your whole hand off, put it back, and test again. Do that five times.