Chinese Yiqiang Laser making horizontal etching bands

Hi,
I'm new to this forum and appreciate any help. I have a large format Chinese Yiquiang Laser. I have been using it for etching out of the RDworks program onto acrylic and mirror. I'm having an issue that laser etches bands that are of unequal power so some parts of the image go through to the other side and other do not. I have tuned the mirrors recently and checked the glide bar. Does anyone have any idea what might be causing this.
Thanks,
Dan