The dual-band LNBFs, such as the BSC621, Ck-1, and DMX741, all are tricky to setup for Ku.
First, their Ku LNB only sees the middle of the dish, not the entire surface.
That cuts performance somewhat.
(your center hole has no effect, as B.J. noted)
The fact that the Ku probes are recessed inside the feedhorn, may contribute to the adjustment difficulty.
From what I've seen, you need to get the LNBF centered perfectly on the dish.
Next, you need to have the LNBF point directly to the center of the dish.
Those who are careful, seem to have good results.
There are plenty of reports by members who have been successful.
However, "close" is not close enough.
If you are unsuccessful, another approach which seems more easily tuned up, is the orthomode feedhorn.
Here are two examples by
ADL and
Chaparral.
Both are expensive, and need two C and two Ku band LNBs, plus a multiswitch.