Reason 1: The focal length is incorrect.
Reason 2: The laser intensity is not adjusted to the maximum.
Solution: Adjust power to moderate.
Reason 3: Water temperature is too high.
Solution: Replace the circulating water.
Reason 4: Laser tube decay.
Solution: Replace the laser tube.
Reason 5: Optical path deflection.
Workaround: Correct it.
Reason 6: The lens are dirty.
Solution: Clean the lens.