If it's for the whole of the class, check to make sure that 1)
- the assignment is actually published (look for timestamp underneath published in upper right hand side), and
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- the assignment start/due date is appropriate.
If both of these look good next step is to generate missing cua (usually for one to a small handful of students, not a full class).
- masq as instructor
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- drill into specific assignment
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- edit settings
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- check that start date is appropriate
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- if students complaining, start probably in the past
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- sometimes wrong year/month is put in here