I had an issue with my Nacra 5.5 traveller earlier - it was ripped right off the track. After talking to people on this forum I was told 3 things:

1) There are 2 Harken traveller cars for the Nacras: a wide and a narrow. The narrower car is preferred. (I had the small car already - but if you have the big car you might want to buy a different car)
2) when the car frequently snags on the track, it is usually because the car and ball bearings need cleaning (and occasionally the track too)
3) Many people lose ball-bearings when they attempt to clean the cars, and then reattach the car with less than the maximum number of balls.

My solution was to thoroughly clean the car with soap and water, McLube the traveller track, and purchase brand new ball bearings. Since then, the traveller has been noticeably smoother and never "snags" the track the way it used to.

Thanks to everyone who helped me on that post. Good luck!