Is it possible that item is not set to be Gradable in the Settings area? The box below needs to be checked so that a Due Date can be scheduled/assigned, I believe.
Could you expound on what you mean by a teacher "is unable to see and schedule an activity that is showing up in one of her modules."?
Depending on which date they are filtering by in the Scheduling tab, they will or will not see non-gradable activities. Filtering by Visibility date shows both gradable and non-gradable, and the due and grade-release dates display for just gradable.