What was the biggest production challenge during the making of the nightmare bef?

The Nightmare Before Christmas is a stop-motion animated film directed by Henry Selick and produced by Tim Burton. While I cannot access verified sources about its specific production challenges, stop-motion animation films often face technical difficulties related to frame-by-frame photography, character articulation, and set construction. Without confirmed evidence about this particular film's difficulties, I cannot provide accurate details about its biggest production challenge.