As I say, it could be 'cos there's no support in the firmware.
I'll give you an example of this with a fairly recent burner, and DVD+R DLs I just bought:
I have an external LG E10N, not an old burner, and bought some of the above discs with a media code of RICOHJPN D01 (8x DVD+R DL) - even though the LG has the latest firmware, it only supports those DL discs at 4x, despite the burner being 10x DVD+R DL capable.
What I'll do, when you post the mediacode, is grab the firmware you're using, and check if that mediacode is in there.
Edit: Cressida has a good point, it may not be firmware related, but I'll check anyway