Response Groups and Not Ringing a Person in the Group

John Haverty  This could be response group sync issue between front-ends. I would suggest on each front-end to filter down Skype event logs to "LS Response group service" and check if there are no errors, especially with 'match making.

Next step can be to check connections to Front-ends for the agents using get-csconnections.ps1 script. Run it for the different agents and check against these who receive calls or don't, you should be able to spot a trend, that the ones receiving or not receiving are connected to specific fronted.

Get-CsConnections.ps1 -SipAddress [sip address] -Pool [pool FQDN]
And finally, you can try to restart Response group service on all PCs at the same time run this on all  front-end servers at the same time

Stop-CsWindowsService RTCRGS | Start-CsWindowsService

Of course this will interrupt all response groups for a short period. If that will not help, try to restart problematic front-end that you figured out with "Get-CsConnections.ps1" script.