1. Your time is not in the top 3000 in the world for that track. If your time is not good enough it will show up as a -. Also not that lap times for player matches (unranked matches) does not register on the online leaderboards.
2. If you know for sure that your time is in the top 3000 your time is not showing because of a bug in Burnout Revenge. This problem affects both the online leaderboards and your best time that is shown when a track is loading. For some players the lap times just disappear or never register at all.
Criterion claims that there are no known bugs with this feature. BurnoutAholics.com can however confirm that there is a bug with this feature and that lap times sometimes just disappear and never show again. If you are lucky your time might have registered on the offline leaderboard.