Optiarc AD 7173A - cannot write more than 4x

i recently bought an optiarc AD 7173A to use it with my external USB 2.0 enclosure. I have tried writting to various DVD-R media but cannot achieve a speed of more than 4X. The media codes of the disks i am using are:
CMC MAG. AM3 (tdk), MCC 03RG20 (verbatim) and the TYG02. Any help is appreciated.

Welcome to the forum.
These drives seem very picky about the chipset used in the external enclosure.
The drive will most likely work perfectly if connected to an internal IDE channel.

Thanks for your response, i also believe that my enclosure is not the right one. Are there any known to work enclosures for this drive?