By default students can go back and modify their answers based on your feedback.
However, there are three exceptions:
You hit "Stop Responses" for the assignment.
The student has exceeded the allotted number of attempts on an autograded multiple choice question.
The student has locked a section.
If any of those three things has happened and you still want to give students access by:
1) selecting "Accept Responses" for the assignment
2) edit the assignment to add attempts to a MC question
If you open the assignment in Grade by Student view you can see who last worked on the lab by looking in the "Last Change By" column as shown below. It will either show that the instructor (you) last left comments, or the student has updated their answer since you've seen it.