diseqc switch and dm1105

this may be a busted switch

i have a dm1105 connected to 4 lnbs via a 4 way switch

i've checked each lnb individually and i can tune to boradcasts on each one when i am not using the switch

however i can only tune to lnbs on the first two positions but not the third and fourth

has anyone else had this problem?
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simon