OK I know "midstart" isn't a great dec=scription...but please help!

I have books on tape and I listen for say 5 hrs....however the book is 10 hrs long...I shut off WMP10 and then want to restart where I left off, however, it won't let me go to 5 hrs in ...but only like 3.5...over and over I try...soft reset, shut it down and try again etc...

TIPS?? ideas suggestions welcome.

thanks!