Students who are struggling with a puzzle and unable to move forward will show an alert on the teacher's class usage page when they have attempted a puzzle 10+ times to indicate that they are stuck.
Students with alerts are indicated by a yellow warning sign next to their name on the far right of the page. If you click on the name of a student with an alert, you can see which game they are stuck on as well as how many times they have attempted that game.
You can also filter students to highlight those students who have alerts. To do this, click the Filter button under "My students" and then select "Alerts".