I'm using Rack Combo B as my default rack set up.  However, I am trying to do an instrument change to Rack A.  Rack A is loading into Kontakt but will not playback after I have made the instrument change.  In Kontakt I can totally change Rack B to A of course that changes all the other sounds to playback with the Rack A sounds not rack b sounds.  Not having a problem with instrument changes regarding keyboards, i.e. xylo to crotales, glock to chimes etc.  I even get an instrument change for rack b to chimes to play back, just can't make a instrument change from rack b to a.  I also am only using 12 channels at the moments so channels are not the problem either.  Anyway, any help would be greatly appreciated.

Ok, I am getting Rack A to playback now, however it is the wrong sound.  When I imput a concert snare shot using the C5 key on my keyboard (korg triton le) it triggers the correct sound, but during playback I get a woodblock sound (the one that is assigned to E2) also when I click the notehead with the mouse it triggers the E2 on the Kontakt Player keyboard and plays back a woodblock sound as well.  I remember reading a thread or tutorial stating that some keyboards use C3 as middle C and some using C4.  Does this sound like my problem?  I don't think it is, because then I would be having the same problem with all my other instruments right? 

When you say you're only using 12 channels, do you mean you have 12 staves in your score? If that's the case, keep in mind you may still have all your KP2 slots filled up if you have other instrument changes occurring in other staves. That may not be the problem, but just wanted to be sure that point is clear. Of course, the easy way to tell is to simply open that instance of KP2 and count how many instruments are loaded into it.

Another thing to check - make sure that in the mixer you have everything set to [b](auto)[/b] (the parenthesis are important here). Some people find it tempting to make manual settings here (like Rack Combo B), but doing so will inhibit Sibelius's ability to perform automatic instrument changes.