Ditto djrolo
I was have ing same issue and just could not get the C chord to sound as welll as the other chords. I saw a video on Guitar Tricks, and sorry do not remember the instructor, but he said if you lower your thumb down to the bottom half of the fret board behing the chord It will clean and those missed notes. When I remember to move my thumb down it works great.
Try it you will like it
Good Luck
Dan