Might be the range?
If you are saying that when you play say the Violin at times some of the notes don't play, then it may very well be the range of the instrument in Edirol versus the range on the midi file.
I know that I use reason and Sonar (sonar has the edirol light version included and reason has the most control over anything else) I have to add the range of the intrument of it isn't hitting all the notes.
I don't use Edirol, but if you can see the notes as the play you might watch the notes play and see if sounds don't play above or below a certain note. You can compensate for this, but I don't know how to do it in Edirol.
I wrote a bit about how I make music using MIDI files in another thread
HERE
Hope this helps