Soloing, revisited

I seem to have a real problem grasping this concept.  I can do it on a trombone in jazz band, but not a guitar.  I don't know why, but can anyone tell me what you do when they go about soloing?  Is it really necessary to know all that stuff about modes, etc? I mean, I know a little bit of music theory, but when you solo, isn't it supposed to be less mechanical and more creative? I can riff and play rythms fine, but when trying to solo, I just can seem to get in that "zone", where you are just playing the notes and everything else takes care of itself.