Ishige was never good enough, Nagasawa showed bits and pieces but never really put it all together, Retre was exactly what he was at previous clubs - an OK A-League player who has good games then shark ones in equal measure
OK A-league players who are dependable squaddies in good teams (like Retre in his previous clubs) rarely work out as key pivotal players in average teams (like Retre at the Nix).
Retre is not a particularly creative player for a defensive mid, but he performed consistently well at Sydney FC when surrounded by creative players.
I think the answer lies somewhere in between: our creative players like Nagasawa are not physical enough in this league, while our physically adequate players like Retre are not creative enough.