What software do you burn with?
I assume that you still get the option to burn at the higher speed but that it is just not achieved.
If it's Nero or DVD Decrypter/ImgBurn etc that show the buffers then watching these can give you clues. If the read buffer keeps dropping significantly then the app can't get the data from the HD quick enough. This could be a DMA issue on the HD or severe fragmentation. To maintain good burning regular defragging of the HD is ESSENTIAL.
If it's the device buffer dropping significantly then it could be down to the media, which is what?