It’s called the ionization point of the tube, and it’s a properly of glass tube CO2 lasers. The minimum firing value tends to be roughly 10% of the wattage of the tube, because bigger tubes need a bigger kick to fire them.
There’s a setting called ‘Engraving mode’ in the machine settings that you can change from ‘Common mode’ to ‘Special mode’ - with this setting, the tube is given a higher power kick for just a moment, and then the power is backed off, because it takes less to keep the laser firing than it does to start it firing. This will let you engrave at lower power, but it also stresses the tube and will reduce the lifespan somewhat.
My suggestion would be to go faster.