Suba is correct. Layer break positions can vary widely, and depending on what software ripped the DVD (and depending on what ahhhhem...undesire-ables were removed) the positions for the layer breaks may have been completely removed from the rip. It is very possible to set your own, however, I do not recommend it because unfortunately it is a huge
pain in the ass. As Suba said,
ImgBurn is your best bet. I have had very few problems, and the ones I did have could be traced to inferior
DVD+R DL media. I now only use Verbatim DVD+R DL media. Here is an excellent site detailing which media sucks big time, and which is worth your green dogs.
http://www.digitalfaq.com/media/dvdmedia.htm
The Verbatims are MKM001.
The other thing to make sure when burning Double Layered discs, is that no matter what anyone tells you, and no matter what speed the discs are rated at, or how fast your burner writes, never EVER burn faster than 2.4X. Always burn at 2.4X. If you don't believe me, there are many forums detailing why this is so.
Further, make sure, if you intend on watching your burn in a set-top or standalone DVD player, that your burner supports bitsetting or booktype-ing, that is, being able to set the media ID of your DVD+R DL to DVD-ROM, so that your player treats it as such.
http://en.wikipedia.org/wiki/Book_type
Actually I looked up your drive and you are lucky. It supports bitsetting, but only for DVD+R DL which is what you say you are using.
http://www.cdfreaks.com/reviews/Sony-AW-G170A-DVD-Burner-Preview
The best advice I can give without this turning into a book, is to read a tutorial on how to use the "Build" mode in
ImgBurn.
Here is the absolute best guide I can find, and the one I used when I was learning:
http://forum.imgburn.com/index.php?showtopic=1779
This should get you started. Good Luck