The problem may be that your laser is not that strong (or the battery is dying). I will guess that your laser is attached to your arbor, where the saw blade is attached. When you first start the saw, the blade is spinning slower and you see the light for longer on the spot it hits. As the blade spins up, you don't get as much light and it looks dimmer. Similar to what you see when helicopter rotors spin up.

There are replacement lasers made that might work better. Irwin is one manufacturer.