There are instances when we see that even though resources are assigned on the tasks, yet in Resource Usage or Task Usage Views you see 0 hrs. assigned to those resource. Refer to the snapshot below, even though resources are assigned yet in Resource Usage their Work Hours is shown as 0.
Workaround : Turn on the Calculation Mode after each edits by -> File -> Options -> Schedule -> Calculate Project after each edit – turn it ON. This will fix the above problem.