[Wapt] Some machines are not reachable in the wapt-console.

We have problems with some machines in wapt-console, and in the column where it normally says "OK" or "DISCONNECTED", for some machines we just have the icon of a clock, see screenshot. The machines are online, and when we reboot them, they get all the packages from the wapt-server they should have. Where and how can we troubleshoot this? On the wapt-server (1.5.1.26 on Debian stretch), or on the windows machine running the wapt-console?

We have 80 machines, and about 3-4 of them have this problem.

We have tried reinstalling the wapt-agent.