I'm using maya 2009 , but i had this problem in 2008, too.
It happens that cache clips in the trax editor don't have any effect even though they are enabled. When I reimport the clip it works, but then other clips suddenly don't . There's no warning or error message, it just acts like the clip is not enabled. No weight curve either. Please help, this is really annoying and i have to get the work done.