First, each tranponder can be transmitting at a different signal strength. They can even be carried on separate beams, with different footprints.
Second, some tranponders carry many channels, others fewer. Those carrying many, especially when they are not using the statistical multiplexing techniques, can sometimes be trying to transmit too much information at once. This will be particularly true of they are carrying multiple sports channels, where the screen content changes rapidly. When the combined rate of information rtequired to be transmitted exceeds the available bandwidth on a transponder, pixellation is the result. Even if the reception itself is perfect. And even with the biggest dish in the world.
accurate alignment of the dish with the satellite is of paramount importance. Note: the bigger the dish, the MORE accurately it must be aligned. So small dishes are less sensitive to alignment (sounds unintuitive but it's true.) Of course, small dishes van only receive strong signals in the first place, so a big dish is always preferable.
Alignment of the LNB is crucial too. It must sit exactly at (one of the) focus of the dish, and the polarisor must be oriented to match the skew of the signal.
I posted this question about 2 months ago about transponder problems. I have since then replaced my dish with a smaller one but am still receiving the same results on certain transponders on hotbird. The Lmb alignment i think i can now rule out etc and the dish size i honestly dont think is making any difference as the dish i had before was very big, certainly big enough for the hotbird (no pun intended The signal i am getting from the transponders causing breaking picture is very good so this picture break up only ever occurs when the signal is weak and goes into the red on the signal strength bar on my pc software. These transponders do change and carry different amounts of channels at different times but i still get the same results. So what can i do?