If the first server listed in the group is not available, it should go to the second in that scenario. fail through is not needed for that. If you wanted fail through to work (go to the second even if the first is up and responding) then you need to terminate on the controller.
run the following from the CLI to see if it gives you any insight into whether it is trying to use the other server; and if it sees the other one as down.
show aaa authentication-server radius statistics