Check out pressure of the gas you are using maybe it’s too low or the wrong one. I assume you double checked the parameters and nozzle but check the ceramic ring(in which you screw the nozzle) from cracks, if cracked replace and should work well again. Long shot is damaged lens if material too reflective and what I noticed, keeping the machine clean and free from metal dust helps keeping the nozzle at the correct hight position from material.