The logic is correct,the frequency's are the same. That means that changeing sats has no effect on the LNB. Other than polarity change at the feedhorn. This means that if the LNB were bad it would be bad on the same transponders on all sats.
You have either an alignment(tracking)problem or an obstruction blocking Your line of sight on that side of the arc. If You are to far east geograpicly this can amplify the problem. If it is a gegraphic problem then a weakening lnb can cause this to some degree as well. The rain is blocking...
