Make sure your GRBL Spindle RPM/PWM are mapped correctly.
Laser Power is correlated to the GRBL Spindle RPM/PWM.
I recommend that you set your GRBL settings to the following:
$30=255 (Maximum spindle speed, RPM) $31=0 (Minimum spindle speed, RPM)
I also recomend looking into GRBL Laser Mode:
$32=1 (Laser-mode enable, boolean)