NEC-ND2510, Nero 6.6 and DVD-R media issues

Hi all, Newbie to group here.

I have Nero 6.6, an NEC ND2510 8 speed dual layer writer. Up to recently I have been using Datawrite Red (V.3) 4x dvd-r media (4.7gb) and can generally get around 4.5gb of data on it before I am told I need to enable overburn.

I recently purchased some BulkPaq 8x DVD-R disks also 4.7gb stamped on them, however when adding files to the disk, I get to around 3.8-9gb and am told I need to use overburn, and this fails at any rate so I can only fit 3.8gb on the disk else nothing is burnt at all.

Does anyone know why this could be the case. I am using the most up to date firmware for the player 2.17 and this makes no difference.

Thanks in advance for any help