Now on to the left hand.
Here's a more obvious problem:
You can see my middle finger is too far from the third fret to get a clear note on the sixth string. I'll either get a dull note or a buzz because I can't hold the string down tight enough to the fret to get a solid contact.
Now, I've got it right:
This example makes it very plain, but lots of chord positions are far more subtle, so it's a good idea to analyze what your left hand is doing if you get that occasional buzz or dull sound.
The same thing happens if you're not strong enough to hold the string down against the fret. Suppose the action is really too high for you to get a clean contact in more complex positions. This is a fairly common problem, especially in the case of a high nut.
Sometimes the frets are to blame.
Here, I can hold the string down to the fret and get a very solid and clean contact:
Notice that my finger isn't touching the fingerboard, and neither is the string.
If the frets have been filed or worn too low, I can't hold the string down to get a good contact with the fret because I "bottom out" against the fingerboard:
In an effort to get a clear note, I'll have to mash really hard, if I can do it at all!
This is a very serious and quite common problem. Many repair technicians appear to be unfamiliar with this situation because they file all the frets really low in an effort to make them all level. The result is an instrument that is very difficult to play cleanly.