As was mentioned earlier, the KDF is very heavy and will lay on top of the CR100 because the media can not be lifted properly when backwashing to cause the KDF to settle to the bottom. Take the end of a broom handle and poke holes in the top of the media (as suggested) then run the system through backwash several times to see if KDF has settled to bottom. Repeat if necessary. We have done this several times with very good success.
You don,t need to rebed the system unless the CR100 has eroded away (due to PH correction) and that should not be the case in such a short time.