David, I spoke to a friend of mine who is one of those mayor eminences in midi stuff (he was one of the ones creating the sound libraries for Roland back when the U and JV series).
Any way, he told me Logic 8 has loads of bugs and it seems this is one of them. He recomended me to go back to Logic 7 that was a more realiable versión.
Still, I know I have done what I want to achive before but is not happening now.