DVD Writer probelms

HI all

I am using Liteon DVD writer
The problem is that some of my dvds are not even detected by it

It's not like that none of the dvds are dectected, but few

These undetectable dvds include Digit's also (Apr 2007, Jul 2007, Oct2007). But the same digit dvds runs fine on my friend's Sony DVD writer.

Also I observed that two of my dvds running fine on my DVD writer (Liteon) are not detected by his DVD writer (Sony)

Can anyone please tell me what's the reason, I am frustated with this problem.

Thanks to all

Lalit.