OK, then regardless of how many receivers you have, I'm assuming the 2 dual tuners only, you can still troubleshoot.
Take the 2+2 cable leads and hook them up directly to the satellite dish, meaning 4 leads off the dish will be paired to each receiver. This setup SHOULD work for those two receivers with no troubles. OR any combination of receivers that you didn't mention. 1 Dual and 2 Singles, 4 Singles, 1 Dual and 1 Single leaving 1 spare, etc. Thing is to eliminate the connection to the "suspected" multi-switch. If this setup doesn't work we can assume the dish.
The 3 headed dish has a built-in multi switch. When you pair that dish with a multi-switch the dish loses its responsibility to switch. Each Leg is now dedicated to 4 different orbital to polarity assignments and the multi-switch is responsible for them. They are 101Left, 101Right, (110Low&119High)Left, & 119Right. This might be good to know to study the degraded signals and compare them to which lead between the dish and multi-switch to look for possible troubles.