If it's for the whole of the class, check to make sure that
- the assignment is actually published (look for timestamp underneath published in upper right hand side) AND
- 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
- drill into specific assignment
- edit settings
- check that start date is appropriate
- if students complaining, start probably in the past
- sometimes wrong year/month is put in here