I've bought secondhand dreamcast and I've had the same issue with two of them.
They don't read certain official games such as : Sonic Adventure 1, Ooga Booga... (while my jap system have no problem reading them).
I've already tried the trick to make the laser read more faster (turn the little screw clockwise), but without luck.
Is this a common issue?
Do I need to change the whole drive or only the laser?
Is there a place where I can buy that part to replace it or I need another console donor?