Why is Moviefactory reading blank DL as single layer?

I have Moviefactory 2, and an NEC 3520A DL burner. I have used Moviefactory successfully with the NEC on Single Layer +Rs. I have just tried a Datasafe DL 2.4x +R in MF2, but it is still saying 4.7GB capacity, not 8.5GB.

I have also set the booktype for DL to DVD-ROM in DVDInfoPro. This program correctly identifies the DL’s capacity.

Does anyone know how I can get MF2 to recognise the DL discs as 8.5GB?