Have you tried burning at 2x speed? A number of forums recommend burning BD-DL (50GB) discs at 2x speed, even if they are rated at 4x or 6x, in order to reduce the chance of things going wrong at the layer change.
Also, I would recommend burning with ImgBurn software and “verify” turned on.
Out of interest, what is the media ID code of your discs? ImgBurn should be able to tell you this.
Bluto
I had my discs scanned with imgburn and it says that the disc id is MEI-T02-001. The max supported speed was 4x, so maybe that could be the problem.