Pixelation is generally related to signal strength, but not always. It can be a cable issue if the cable is damaged, the wrong type, or has an intermittent short.
To understand how a multiswtich works, you need to understand how the receivers select the channel you are watching. Each receiver sends a signal back to the LNB on the dish. Based upon what channel, and therefore what satellite/transponder, you are watching, it will send back one of 4 signals. For channels that are one the 99 and 101 sats odd transponders it sends back 13VDC. For 99 and 101 sats on even transponders, it sends back 18VDC. For the 103, 110, and 119 sats, it uses the same voltages as the 99/101 sats but adds a 22kHZ control tone to the signal.
What the multiswitch does is take those 4 combinations and lock in the 4 cables coming from the LNB assembly to each one. The switch then takes the signals coming back from the receivers and routes them to the appropriate leg on the input side of the switch to send the appropriate stream back to the receiver.
In order to rule out a switch issue, you can temporarily bypass it with a 3ghz barrel connector plugged into any one line from the input and the output going to the affected receiver and see if the problem clears itself.
You could also post your signal strengths on all sats/transponders, to see if you need a dish alignment.
Replacing your switch with a 6x16 is easy, although you will have to run an additional coax back to it for power. You could also daisy chain to 6x8's to give you 12 outputs or install a second 6x8 in parallel.