agi:
Try switching cables. You most likely have a wiring problem. A DBS LNBF uses differences in voltage to determine odd or even transponders. 13V for odd, 18V for even. If you are not getting even transponders, that means the LNBF is not getting 18V. A bad cable end, water damage, too long of cable run, staple in the cable, or using RG59 are all possible causes.
On a related note, a multi switch does take one side of LNBF and make it odd full time, and the other side even. This allows you to connect more than 2 receivers to a dual LNBF, via the switch. As you only have two receivers, a switch is not necessary, because the particular receiver you are using at the time (either DISH or VOOM) sends the needed voltage to the LNBF. It does not matter that one receiver is DISH, and the other is VOOM. The only thing the LNBF "sees" is the voltage from the receiver.
If you are receiving all transponders on your VOOM receiver (VOOM only uses one even transponder), try connecting your DISH receiver to the LNBF with the VOOM cable (or run temporary cable), and verify even transponders. If you are now receiving even transponders, then you need to replace DISH cable, or find the source of voltage loss.
One more thing. If you had your 301 connected to a twin, quad or switch of some kind prior to now, go to the "check switch" screen in system setup, disconnect the cable to lnbf from receiver, and run check switch routine to "clear the matrix". If the receiver is looking for a switch that is not there, this may also be causing the problem.
Good Luck
Alan