Plot hole: In this film Chucky says they need the heart of dumbbells to transfer their souls into human body. If he knew this why did he even attempt it in the first 3 movies? And he never uses it in the next 3 movies, including "Cult of Chucky," where he is successful at the transfer. (Side Note: The series writer has admitted that this was a ret-con added to the film in order to justify the road-trip. So, while still a mistake, it was purposely done).
Suggested correction: The amulet is necessary to amplify power in order to transfer multiple souls at a time - it's only used in Bride with two souls and Seed with three or four.