IIRC if you're missing several channels then half of your LNB is most probably buggered (given that they have a horizontal and a vertical plane for channel reception).
Also, I found it useful to put Sky on the service menu showing the signal strength and quality, and get up a ladder to adjust the dish whilst watching the outcome (noting that it takes a good 3-4 seconds for the result to be displayed). I now have an almost perfect signal strength and quality from a standard dish.