First thing to do is check the DVD burner you have in your Mac to be sure it supports dual layer DVDs. (Dual layer DVDs have a capacity of 8.5 GBs, as opposed to 4.7 GBs for a single layer DVD.) It’s unusual for a burner to stop burning one type of DVD but still burn others, so I’m wondering if you really have a dual layer burner.
With all the different types of burners and DVDs available, this can be confusing, so the first thing to do is be sure of what kind of burner you have. Go under the Apple menu to “About This Mac.” In the pop-up, click on “More Info…” That will take you to the System Profiler.
In the left-hand pane, find “Disc Burning.” Click on that and you will get all the information on your disc burner in the right hand column. You’ll see the brand name of the burner, the firmware and other hardware details. Look for the DVD-Write line, and see if it lists “-R DL” and “+R DL”
If it does, that means you do have a dual layer burner (the DL) and it will burn DL discs (the -R and +R). So maybe the problem with burning dual layer discs is a bad disc or batch of discs. Try other discs and see if one will burn OK. If you don’t see “DL” in the Disc Burning line, than you don’t have a dual layer burner, and maybe the DVDs you made in the past were single layer.