Feedhorn placement is hyper-critical for the higher frequencies of KU band. It becomes even more critical for the combo C/KU feedhorns. If the signals are not converging and reflecting inside of the feedhorn cavity at just the right angles, the signals may arrive at the probes out of phase. I have found that it often takes many minute adjustments while comparing C and KU meter readings to find the perfect balance to optimize both C and KU bands. With that said, some dishes FD / FL designs are better than others for balancing the C and KU performance. Some hobbyists have experimented by moving the KU section away from the feedhorn by inserting spacers between the two sections. This fine-tuning technique places the KU probes in better position to capture the reflected signals.
Here is a link to to the install sheet providing instructions and calculations for the scalar and feedhorn settings:
http://titaniumsatellite.com/downloads/Guides/CK1S_LNBF_Install_Guide.pdf
What is the diameter of your dish from edge to edge of the parabola? What is the depth of the reflector?
Is the scalar placed at the correct distance? When the feedhorn is set to the calculated FD ratio stamp, is the focal length measurement correct?
Additional Information: The FL measurement made 1/4" inside the feedhorn will often provides better performance on a shallow FD ratio reflector when this point is moved deeper into the throat (moving the feedhorn closer to the reflector).
On my personal I use a dedicated C-band LNBF on the BUD and a separate 90cm dish for KU. I am not a fan of combo LNBFs, but I will carry the best performance model of the combo C/KU for the folks who want them!