You may face an issue where EndPointSecurity is not updating the connected devices list.
This can be seen by going into Activity > Log Browser > Device connected events and you find that the last update you have was not recent.
This issue is due to using either a wrong or expired license key which causes agents running in freeware mode.
The solution to this problem is updating the license key you're using to a correct one. It can be done by going to General > Licensing > edit. You can find the current license key by following the steps in the article Obtaining Your GFI License Key.
After you update the license key, try to deploy the agents again and check if the issue is resolved by connecting a new device to the controlled machine.