Brush, shake any foreign material off before sending down the belt.
Another thing you can do is to encapsulate the pieces by spraying them with tempered cocoa butter before enrobing.
I learned this technique from someone who was filling shells with fruit preserves - the chocolate bottom would not stick to the preserves, leading to all sorts of problems. Spraying the exposed surface of the preserves with the cocoa butter and letting them sit for a few minutes solved all of the production problems.
I can imagine it being used to keep powders and very small bits stuck to the surface of what's being enrobed.