The gateway, netprobe and license daemon logs should be checked for error messages or clues if this happens. This article talks about additional checks that can be made.
If you notice a warning triangle with an exclamation mark on your entity - like below as we as checking the logs as above you can click on the entity for further information.
Example if we click the entity PG - you can view the details of the Entity in the Managed Entities view - In the below - you can see a disconnected icon, this means the NetProbe is likely disconnected or down.
There may be a scenario where the netprobe is running but the exclamation mark still shows below - you have clicked on the managed Entity and it is showing blank or not all the samplers you expect to see
You can right click the entity and choose Properties - this will give you more information.
Always check the large icon - and any wording with Licensing this gives you further details. In this case it seems like the NetProbe is not licensed.
As you can see the license daemon has disconnected / is down. This will need to be restarted in order to fix the issue.
More details on setting up Gateway-licenseUsage plugin below.