Hey why does flatten command stop working

It worked fine for a month or so now the drawing just reaches a point where it stops flattening if I scribble next to it it flattens that though so I’m confused I usually smooth before I flatten so it’s not that.is there some way to delete your history in toonboom ? What can be causing this?

Also copy pasting the lines to another layer doesn’t solve the problem .Plz help

Figured it out I had 2 blacks from different pallets :slight_smile: