Here's my first attempt at a shaving cream/splatter dye. I did the splatter in the center. I did it with liquid dye instead of straight powder, mostly because I mixed all the powder dye into liquid(my mistake). Pretty happy with this outcome minus the white in the middle. Maybe I didn't put enough dye in the middle?
I've had similar happen. If you don't have dye down it won't dye obviously, but I've found that the non-dyed spots are usually from lack of contact with the bed of cream (air bubbles between the cream and the disc). Whether the cream is too thick or not even across it's top.
A couple things to help. You can make the cream mix thinner. It can be super thin and still float a disc. Also, take your time and lower the disc in super slow to avoid the air pockets.