That is really bad. How fast of a shift are we talkin'? I think the problem might be in the synchros. If you shift slow and it works but does it on a fast shift, it is definitely synchro related. You might try a different tranny fluid next oil change. I know redline advertises that their fluid optimizes synchro performance for aggressive driving. Else, Slow Down!
2001 Sentra SE w/PP