I've come across this too.robburge@sky.com wrote:much better at lower power
The explanation I've been told is that, when you blast some materials with full power, the excess heat can 'bake'' a layer of very dense carbon etc. into the bottom of the kerf - which the laser then finds very difficult to penetrate any further, even if you try cutting in two passes. At lower power, the kerf gets a chance to vapourise away without the uncut material beneath getting 'baked', and the laser only has to cut through nice 'virgin' material.