This is a known problem in Flexi and Photo Print if you print with FMXPress Diffusion. Go to RIP and Print, click on the Advanced tab, if you have the latest version it will be an icon with three ink drops on it, and change the Dither Type to Error Diffusion or Error Diffusion 2. For best results use a profile that was created with Error Diffusion, Error Diffusion 2 or SO Diffusion.
Chris Philipps