Yup, CD-RW media are keyed for a certain speed and do not allow burning above that speed. While CD-R media can be burned (with varying success) at a speed higher than the design speed, CD-RW media do not allow this. The majority of CD-RW media that I have found is still 4x, though you shouldn't have truoble finding higher speed CD-RW discs if you look a bit. More than likely this is the problem.
Zenmervolt