Cite by Titanium on old post on SatelliteGuys.us: The KU LNBF's are designed for .5/.6 FD offset reflectors, so it is only seeing (illuminating) a portion of the reflector surface.

If you want higher gain, use a prime focus LNBF, which is designed for illuminating a .3/.4 FD reflector (C-band dish). Then it will see the entire reflector surface. Geosatpro sells the SL1P model, which is prime focus.
Code:
https://satelliteav.com/sl1p/
For even better KU performance on your BUD, use an Invacom C120 feedhorn and an Inverto Black Pro flange LNB.