The progress bar depends on the types of activities included in the assignment.
Some activities are designed solely for practice purposes and do not generate a score.
For this reason, even if the student has completed all the activities, the progress bar may not show 100% completion.
These non-scored activities will eventually be locked so they do not interfere with the formal evaluation of an assignment.
Suggestion: Check that the activities within the assignment are gradable if you want them to contribute to the total progress.