I think your temper is off. Merely cooling and stirring in a bowl over 15 min might not be enough to create the stable cocoa butter crystals. My understanding is that tabling on marble cools it so rapidly that it creates crystals, a bowl isn’t going to be that fast.
Sounds like you’re dipping centers. How long do they take to set? How warm is your room? It might help to refrigerate the pieces for 5-10 min to help them solidify and cool quickly. Or lower your chocolate working temp by a degree or two, streaky pieces can mean the chocolate was too warm.
