Teaching with Moodle

Good morning.
Yesterday, some students have reported that they can not change their tasks which have been evaluated with hepatic, because it lacks the EDIT button. In practice we can only do the task the first time but after the delivery is no longer possible to return and proofread. We do not understand why, this is a new problem.
It could be a bug? The site has the latest update.
Someone said it might be due to some extension installed or deleted .... Help! Can you help? thanks
Nadia