how to recover fee for designs used by clients

I am an Architect practicing independently from home. Last year I had difficult clients. I had agreed to do the design & Drawings and stage wise visits and quoted my fees accordingly. Two of them have cheated me by using all my drawings but when asked for remuneration earlier they postponed & now denying giving excuses, when all work is over. I request you to help me out to recover my remuneration for the work already done.