What would cause Resource Cost to not be included in WIP Completion Cost?

We have a situation where one WIP job had 4 Completions, for the same quantity each. Each Completion should include the same Resource cost but they don't:

WIP Completion 1 has Resource Cost 100

WIP Completion 2 has Resource Cost 0

WIP Completion 3 has Resource Cost 200

WIP Completion 4 has Resource Cost 100

Resources were applied to the job before WIP Completion 2, yet their cost went to WIP Completion 3 and I need to understand why so we can find and fix any future occurrences.

Regards,

J