You have the same problem as me. The Dreamcast's laser unit has worn out. This is simply a wear-and-tear issue. Tweaking the laser pot setting might help, but if done wrongly it can permanently damage the laser. If you can find a replacement laser, it should be able to work properly again. Way back in 2001, it was common to find replacement lasers in Sg Wang, but now...

The tweaking can be done simply by using a precision screwdriver. You just need to tweak the laser pot to increase the power output. However, you cannot simply just tweak it blindly, or else the power output might get overloaded and blow out. If if happens, the laser will be permanently damaged, and you won't be able to use it any longer.