Is your dish damaged in any way?
Can a pencil pass through the mesh holes?
If you have c/ku mesh and the dish is in good shape the feed may not be centered or the fd ratio is off or the feed is not postioned well for the polarity's to work good.
When you are on a channel that is coming in on G-16 (X4) leave the quality meter on the screen, have someone go outside and push the dish softly down see what happends then have them push it up, if the quality goes up your dish still needs tuning.