I have been having teh same problems for some time now. When I assign a task for my student and sheer it with them, a few students are able to do the task with no problems but others, when they open it, they get the message that the task is not available anymore.
Those tasks are usually linked to a timed exercise, so I wonder if it has to do with the time limit. Perhaps when they access it they have run out of that time? Perhaps if the open the task once the are not allowed to open it again??
Looks like an issue with the time limit. Remember that if students share the same device it will count as if they already consumed the assigned time to do the worksheet.
If this can't be the issue, please tell us in which task are you having problems and we'll take a look.
