I ran into the same problem too. A likely issue is that your PRTG machine is using a self-signed certificate and the its failing on those machines. Go to the configuration page for the PRTG plugin, press F12 on your browser, hit refresh and then the button to save your config and look at the network output. You will probably see a self-cert error like in the pictures below (one shows the issue in edge, the other in firefox):
On the workstation having issues, go to your PRTG sever and go through the “unsafe site, proceed anyway” routine and your PRTG sensors should show up.