Hi Joan,
If you are a teacher in a class the only situation I can think of which would allow you to edit some assignments and not others is if your school is using a feature called Blueprint to push out course content from a Master course. This feature does allow items to be locked at the master course level, which would then make them not editable in the course. And the locking is done on individual items so that could explain being able to edit some but not all assignments. If this is what is happening when you open a locked Assignment it should say across the top "locked for ......". You might need to check with our local Canvas admins about this.
If you are talking about Assignments in different classes; it's possible that an End Date or term end date could be preventing you from editing items in an old course. Again, you'll want to talk to a local Canvas admin if that is the problem.
Rick
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