As with all Korg keyboards - when you switch to combi mode, you need to set up the effects all over again.
You can do it either by selecting "Copy from program", and then checking all the
IFX/MFX boxes so the keyboard does it all for you - or you can do it manually
by editing each efect on your own.
What you have to keep in mind is the fact that in program mode, ONE sound can
have up to 5 IFX effects on it's own, while in combi mode those 5 effects are
divided between 8 sounds.
You can choose not to route a specific part of a combi through an effect though.
So keep track of which sounds and which effects are used.
For example - if you're trying to create a piano/string combi... copy all effects
from the piano sound - add strings without the preset effects (because at this
point it
will overwritte the piano effects already copied).
Then route the strings through some reverb which the piano has.
Having hammond combis may get a bit tricky though since it most certain uses
a leslie effect, choruses, maybe pans, and surely amp simulations.
If you're mixing the hammond sound with some other - you will most likely be
doing it with a sound that doesn't use those effects. In that case you will have to
compromise by either not having the effects on the rest of the sounds,
or giving some general MFX that won't affect your hammond that much.
Good luck