some suspicions:
From reading all the threads on the subject, over the years, I have some ideas.
- placement of the LNBF to get C-band is less critical than Ku
- getting the C/Ku LNBF at the exact focal point is quite difficult
- many users seem to put up with "sloppy", when "precise" is called for
- folks are lulled into a false sense of success once C-band signals are received
- the design and construction of existing dual band LNBFs is a real compromise
I always was confused that many who've been running C-band on 1.2m dishes often had a separate Ku LNB beside the C-band.
At first, I thought it was just that the BSC-621 LNBF's were the problem.
Most who put them on BUDs had Ku problems, as well.
Since C-band worked, some abandoned Ku.
Others got Ku to work, and were quiet as to how... maybe they were old time installers who were more skilled...
Linuxman documented the GeoSat CK-1 running both bands on the 8½' Birdview.
(more recently, the DMX741 as well)
- superior dish?
- superior LNBF?
- superior installer?
Yes. Maybe. Unquestionably.
Many mounting brackets put the LNBF near, but not
at the focal point.
The LNBF needs to be aimed at the proper spot on the dish.
I suspect the fact the Ku LNB is deep inside the LNBF, makes aiming it at the dish, somewhat like threading a needle.
Suggestions:
Try measuring the LNB location horizontally. See that it's centered.
See if the elevation markings on your dish are still accurate (suggests the LNBF is high or low)
Carefully peak the LNBF signal on Ku, by adjusting it toward and away from the dish.