D7 chord - headache

hassleham

To start with try to make sure your thumb is placed on the back of the neck on an imaginary line down the middle (as opposed to sat on the top). You may also want to try sitting in classical position (google it).

Both of these things will help your wrist to move to a better position and allow your fingers to arch over the strings and, when you're pressing frets down, help your fingers to sit closer to perpendicular to the strings/fretboard. This will help you use your finger tips (the harder bits of your fingers) rather than the pads of your fingers, and hopefully reduce the amount of buzz/muting.