Check your wiring and make sure that the Sat C kit is connected to the correct voltage output from your multiswitch. Regular LNBs switch between right-hand circular polarization and left-hand circular polarization by internally selecting the appropriate signal according to whether the receiver is sending either 13 or 18 volts to the LNB.
I think the Sat C LNB requires 18 volts, but I dont have the installation instructions at hand to verify this. Unless you are using a powered multiswitch, your multiswitch has to pass the voltages from the receivers to the LNB's. The multiswitch contains steering diodes to keep your receivers from shorting into each other, but it then begs the question as to what the LNB's see at their ALL of their inputs if all of the receivers are tuned to the 13 volt powered LNB outputs.
So are the unused LNB outputs fed no voltage, or do they all get the same voltage? It would be rather easy to measure. In any case, if you imagine having only one receiver and it is tuned to a sat C channel or a 119 channel that goes through the sat C combiner - then that single receiver is having to power TWO LNB's at the same time. Maybe your receivers cant deliver enough current with enough voltage to drive both LNB's? Of course if you have two receivers selecting the combiner output at the same time, then they would share the load and thus be able to easily drive both LNB's.
This could also happen if you have a high resistance somewhere, like might be caused by poor cable connections or a defective multiswitch. What brand is your receiver? If that brand cant drive the load of the paralleled LNB's, then that would be a brand to avoid.
. I'd check everything, but mostly check the connections and voltages first.
