Ever since i defeated the celestial champions, I haven't been able to fight the shadow pieces on a new moon. None of the sculptures shake on a new moon... This has prevented me from being able to get the heart and fight fuel weaver to reset the ruins. I tried it on two different new moons and encountered the same issue.
Update after 3 new moons the shadow pieces finally shook and was able to initiate the fight
Wait til new moon and have the shadow piece sculptures built. They won't shake so you can smash them and begin the fight