Problem with playing DVD in WMP11

I'm trying to play a DVD I burned, but while the DVD is playing I cannot move
the "progress bar" forward to move forward in the video. The fast forward
button works, but I'm unable to slide the progress bar forward to advance to
a later point in the video. I'm able to slide the progress bar forward on
some of my DVDs, but not others. This happens with both DVD+R and DVD+RW
discs. Why is this? What do I need to do to be able to manually slide the
progress bar forward while playing the DVD? Does it have something to do with
the DVD itself?

I'm using Windows Media Player version 11.0.5721.5268 on Windows XP Home
with Service Pack 2.

Thanks.