RPC isn't called when windowns lose focus

As far as I know, RPC is UDP (correct me if I'm wrong :P). My situation is server call a RPC function but client windows is losing focus at that time. Eventually, when client windows get focus back, RPC function can't get executed.

Is there any ways to force client execute RPC function even it losed focus ?

Note: without losing focus, client works as expected. Option "Run In Background" is checked in Player Settings

Thanks in advanced.