Hub log is full with below error  and  Robots are not reporting ..

 hub: nimSession - failed to connect session to HUBIP:48001, error code 10055
 hub: nimSession - failed to connect session to HUBIP:48001, error code 10055

The possible reason is 

if too many  network connection open from hub side  to communicate with robot , then  it get stuck as  ERROR CODE: 10055 is basically a network related issue .Thats why when hub restarted initially it works fine , then when all connection establish issue start occurring again.

run netstat -an|findstr "<enter filter parameter here>'' command to check how much HUB related connection establish. 

Release : 9.0.2

Component : UIM - HUB

ERROR CODE :10055 is a network /system  setting related issue :

  As If you try to set up TCP connections from ports that are greater than 5000, the local computer responds with above code and
"An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full."

Below Microsoft KB can help to change the registry value 

https://support.microsoft.com/en-us/help/196271/when-you-try-to-connect-from-tcp-ports-greater-than-5000-you-receive-t

Proper UIM sizing :

Please refer UIM sizing guide to check how much robot should report to a hub
https://techdocs.broadcom.com/us/en/ca-enterprise-software/it-operations-management/unified-infrastructure-management/9-0-2/installing/pre-installation-planning/ca-uim-sizing-recommendations.html#concept.dita_f4b87772767eef37070b35c09f690b065cb97ea4_CAUIM902SizingRecommendations