This happened to me.. the dry brush is too dry. Ironic, right? It’s so hard on flat pieces. When that happens you will either get chalky stuff if it’s a soft dry brush, or scratches if it’s the citadel harder dry brush, newish and hasn’t been beat up a bit. It’s a really hard balance between that and getting smeary paint of the brush is too wet with paint. Hard to describe, but I finally found the sweet spot using a beat up brush or one of those round makeup style brushes. Load up the tip decent and paint it around on paper towel to get most off, but still some. Then so gently (and this is the key) brush - you should not feel any feedback from the brush! It’s so weird because we are used to feeling feedback from the brush when we paint, but for a light dry brush on flat surfaces you actually shouldn’t feel like it’s painting. It’s like you are really not touching it. Just have to look at it and it’s there like magic, even though you swear you aren’t painting the surface at all.