Check if the job has any critical errors. Go to the Jobs tab and at the end of each job row, you should see a red or yellow box with a number. The red one means the number of critical errors on this job. You can get more information by opening the job details and pressing the info button next to each node.

Once your resolve the issue, you can press the retry or retry all nodes button, which will force the render nodes to try again.

Another possible cause could be that the computer lost connection to the distributor device (Admin tab > Distributor settings) and none of the other computers stepped in to replace it. If you see No device as the selected item change it to any of the computers.