Problem 150 watts laser machine

Dear friends ,

I have a 150 watts CO2 laser cutting machine,

Searching the youtube I found out that other LAser owners 150 watts can cut from 0.1 to 1.5(2mm) steel sheets ,

I tried using Oxigen but i can;t cut it as I saw over the youtube .

Could it be the focal lens ? I use a 20 mm diameter with 63.5 mm focal lengh .

could it be a higher lens with a bigger focal lens .

I look forward to recive your opinions .

regards,

Costin.