Agree with brad. There should be no separation. Your milk powder - if it is whole milk powder - will have a fat content between 26-28.5%. DO NOT RECONSTITUTE IT to make chocolate. Use it dry.
Assuming 54% fat nibs and 28.5% fat milk powder, your recipe = ~35% total fat -which should be fluid enough to pour. If it's not fluid enough for your liking, fluid lecithin at 0.3-0.5% can be added at the end of your grinding - use more and it will get more viscous. If it's still too thick, it's almost always because you've added water somewhere (either intentionally or via ingredients), you've gotten it way, way, way too hot somewhere along the line, or your grinder is a super grinder and you've managed to grind your particles to sub-micron sizes (i guarantee you this is not the problem).
Exactly what nibs, sugar, milk, and lecithin are you using (ie brand names, which country they originated from, or where/who did you purchase them from)?