Complex sentence: But, if a member state has failed to implement a Directive, a citizen may not be able to bring claims against other non-state parties, but can sue the member state itself for failure to implement the law.
Split into simpler sentences: But, if a member state has failed to implement a Directive, a citizen may not be able to bring claims against other non-state parties. It must instead sue the member state itself for failure to implement the law.