2 possibilities:
1-the audio you are trying to fade has nothing past its visible edge. How do you know if there is nothing past the edge? Use the trim tool and try to drag the clip edge(to make the clip longer). If you get nothing, then there's nothing to crossfade(it took me a while to get my head around this, so don't feel bad if you have to think about it a bit
)
2-trashing prefs is worth trying for any "odd behavior"
