What brand of burner are you using? It sounds like a Plextor or a Lite-On. With Plextor, it has a feature called Power-Rec which checks the media and burns at a speed that has been tested as the optimal speed for that media. I can't remember what the feature is on Lite-On drives, but they have a feature that is exactly the same. To write at a faster speed, you'd have to disable these features either in your burning software or drive utility software that came with your drive. (Generally just "un-checking" a box next to the "enable Power-Rec" option in the software.)
This is what it sounds like to me. If you've got a different drive, post what it is and somebody else might be able to help you. Sorry I couldn't be more specific as where to find the options I'm referring to, but I'm in a little bit of a hurry... But you should be able to find them with a minimal amount of digging through the options.
All of this said, though, I would not reccommend disabling this feature. Especially if your media is questionable. You are going to be getting much poorer writes if you try to burn at 40X+ if Power-Rec is telling you to only burn at 16X.